MongoDB index is not being used - mongodb

Question

I have a collection of questions with index on modified.
{
"v" : 1,
"key" : {
"modified" : 1
},
"name" : "modified_1",
"ns" : "app23568387.questions",
"background" : true,
"safe" : null
}
But when I query the questions with modified field, mongo does not use this index.
db.questions.find({modified: ISODate("2016-07-20T20:58:20.662Z")}).explain(true);
It returns
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 19315626,
"nscanned" : 19315626,
"nscannedObjectsAllPlans" : 19315626,
"nscannedAllPlans" : 19315626,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 384889,
"nChunkSkips" : 0,
"millis" : 43334,
"allPlans" : [
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 19315626,
"nscanned" : 19315626,
"scanAndOrder" : false,
"indexOnly" : false,
"nChunkSkips" : 0
}
],
"server" : "c387.candidate.37:10387",
"filterSet" : false,
"stats" : {
"type" : "COLLSCAN",
"works" : 19624020,
"yields" : 384889,
"unyields" : 384889,
"invalidates" : 3,
"advanced" : 0,
"needTime" : 19315627,
"needFetch" : 0,
"isEOF" : 1,
"docsTested" : 19315626,
"children" : []
}
}
When I use hint(), mongo throws an error bad hint.
I have another collection of folders which has exactly the same index and the query uses the index. (returns "cursor" : "BtreeCursor modified_1" for explain())
What could be the difference between questions and folders? Is it possible that the index is "broken" even though getIndexes() returns it? If so, what can I do to fix it?

Answer 1

It seems your index is not completely built in background. You can check this by using db.currentOp() command:
https://docs.mongodb.com/v3.0/reference/method/db.currentOp/#currentop-index-creation
Also check the mongod.log to see any error on the index building process.
The simple way to fix is to drop the index and create it again