Eclipse Mars - imported projects don't show up in Project/Enterprise Explorer - eclipse

Question

I currently have 48 projects in my workspace. All are cloned git repos that had existing projects that were then imported into Eclipse.
The last three repos that I've imported into Eclipse have imported and migrated successfully but don't show up in either the Project or Enterprise Explorer views. I can see that they exist in the .metadata/.plugins/org.eclipse.core.resources/.projects folder.
I can search the projects and create a working set, and I can add the EAR files to the server, and I can open the files, but I can't do anything that I would normally do from the Project Explorer, including viewing their properties or deleting them.
I had thought that Eclipse might have an upper limit on how many projects could be viewed, but my co-worker says she has 107 projects currently in her workspace.

Answer 1

Maybe the projects are hidden by an active Working Set or by Focus on Active Task.
From the first link:

Answer 2

How to change Eclipse's "Project Explorer" "Filters and Customization" view settings to force projects and files and folders to show up and not be hidden, such as nested projects
Tested in Eclipse Version 2021-06 (4.20.0), Build ID: 20210612-2011 on Linux Ubuntu 18.04.
Quick Summary
Go to the "Project Explorer" tab, click the 3 vertical dots in the top-right corner of that pane, then go to "Filters and Customization...", as shown here:
Uncheck the box for "Nested Projects: hide top-level project if shown as nested" and "Nested Projects: hide folders when project is shown as nested", and click "OK":
Details
I'm having this exact problem in Eclipse Version 2021-06 (4.20.0), Build ID: 20210612-2011 on Linux Ubuntu 18.04.
It turns out it's because of the Project Explorer "Filters and Customization..." settings, as I learned from #howlger's answer.
After adding a code repository directory as a project, I can see it showing up in the deprecated "Navigator" tab, shown as "Navigator (Deprecated)" in Eclipse here (I have blurred images for privacy):
However, it does not show up in the "Project Explorer" tab, which is shown below!
The problem is that the "Project Explorer" tab's "Filters and Customization..." settings are filtering out this project and not showing it. In my particular case, that is because the project I am adding is a subrepo within another git repo, which means it is a project nested within another Eclipse project, and my view settings are mistakenly set to exclude all "nested projects". Your case or reason may be different, but in either case, if your project is not showing up in your "Project Explorer" tab, then it is likely because your "Filters and Customization" settings are set to exclude it from view.
To fix this for my case, go to the "Project Explorer" tab, click the 3 vertical dots in the top-right corner of that pane, then go to "Filters and Customization...", as shown here:
Next, click the "Pre-set filters" tab, then scroll down and uncheck the box for "Nested Projects: hide top-level project if shown as nested". I recommend you also uncheck the box for "Nested Projects: hide folders when project is shown as nested", but I'm not 100% sure what that one does, as unchecking the other box alone caused my project to show up. Here is a screenshot, with the appropriate areas highlighted in yellow:
Any of the items in this list will be hidden from view in the "Project Explorer" tab if their box is checked. So, go through the list in detail and uncheck anything else you might see fit, if applicable. I took a look and left all the rest of my view settings there as-is.
Click the "OK" button when done, and voila!--my nested and previously-added project now shows up in the "Project Explorer" view!
NB: due to some sort of bug in Eclipse, you may have to repeat the above process every time you close and reopen Eclipse. :( But, you can probably do it faster now from the "Recent Filters" section of the 3-dots menu.
Update: it looks like changing the "Projects Presentation" setting from "Hierarchical" to "Flat" blocks this bug and prevents you from having to fix the "Filters and Customization" setting every time you re-open Eclipse.
Here is how: click the 3 vertical dots menu in the "Project Explorer" window --> "Projects Presentation" --> choose "Flat", as shown below:
Other references
Here is the relevant official Eclipse documentation on this: https://help.eclipse.org/: Showing or hiding files in the Project Explorer view:
Showing or hiding files in the Project Explorer view
You can choose to hide system files or generated class files in one of the navigation views. (System files are those that have only a file extension but no file name, for example .classpath.)
On the toolbar for the Project Explorer, click the Menu button Menu button [3 vertical dots] to open the drop-down menu of display options.
Select Filters and Customization..,.
In the dialog box that opens, select the Filters tab and then select the checkboxes for the types of files that you want to hide or define your own name-based filters in the User filters tab.
In addition, you can restrict the displayed files to a working set.
On the toolbar for the Project Explorer, click the Menu button [3 vertical dots] to open the drop-down menu of display options.
Choose Select Working Set...
Select an existing working set from the list or create a new one by selecting New...
My Q&A on how to blur images in GIMP: Graphic Design: How do I do a privacy blur on a section, selection, or region of an image in GIMP?
My personal Eclipse setup document: Eclipse setup instructions on a new Linux (or other OS) computer: Troubleshooting