jQuery .each function across browsers (works in ff, ie8, not ie7) - jquery-selectors

Question

I've been messing with this for far too long, and managed to get IE8 working, but IE7 has me stumped.
I've got a table, and for each column, I am trying to extract a number of divs. I am only extracting divs which match specific selectors, not all divs in the column.
My original jquery selector was
jQuery('div.a1, div.a3, div.a4, div.a7','table#a'+tableId+' td:nth-child('+columnNum+')').each(function(){
alert(jQuery(this).attr('id'));
});
This worked great in FF, but didn't trigger the .each function at all in IE.
After messing around for a bit, I got to
jQuery('td:nth-child('+columnNum+') > div.a1, td:nth-child('+columnNum+') > div.a3, td:nth-child('+columnNum+') > div.a4,td:nth-child('+columnNum+') > div.a7', table#a+'tableId).each(function(){
alert(jQuery(this.attr('id'));
});
Not so nice, but works in IE8.
I had tried all sorts of combinations using .eq(+'columnNum+') but nothing else was working.
Now I go and test in IE7, and again the .each isn't being triggered.
What is the nicest way (and cross-browser compatible) to work with this sort of .each element?
--------------addition--------------
After further testing and playing around with suggestions from DrJ and bdukes, I've found that the table#'+tableId breaks the function in both IE7&8.
I've gone back to my original code
jQuery('div.a1, div.a3, div.a4, div.a7','table#a'+tableId+' td:nth-child('+columnNum+')').each(function(){
alert(jQuery(this).attr('id'));
});
as that seems to me the most efficient.
If I remove 'table#a'+tableId, i get the correct response in all browsers, except that it is adding up the results from all tables, and I need to be able to get only the results from one table at a time.
I have also tried 'table#a'+tableId+'>td:nth-child('+columnNum+')').each, but that doesn't work either.
The first function i've used works perfectly in firefox.
----------------the html being selected---------------------------
The tables are being created dynamically in javascript so I can't really copy and past it, but here is what the output looks like. It ends up looking kinda like a gantt chart on a table.
<table id="a1">
<tr>
<th colspan="5">
Group Name
</th>
</tr>
<tr class="rowId1" >
<td>
<div class="a1" id="a43" style="margin-left:13px; width:60px" ></div>
</td>
<td>
</td>
<td>
<div class="a3" id="a93" style="margin-left:4px; width: 80px" ></div>
<div class="a2" id="a94" style="margin-left:4px; width: 30px" ></div>
</td>
<td>
<div class="a1" id="a24" style="margin-left: 15px; width: 65px;" ></div>
</td>
<td>
</td>
</tr>
</tr>
<tr class="rowId1" >
<td>
<div class="a7" id="a24" style="margin-left:10px; width:60px" ></div>
</td>
<td>
<div class="a2" id="a15" style="margin-left:14px; width: 22px" ></div&gt
</td>
<td>
;
<div class="a2" id="a105" style="margin-left: 8px; width: 50px" ></div>
</td>
<td>
</td>
<td>
<div class="a4" id="a102" style="margin-left: 5px; width: 45px;" ></div>
</td>
</tr>
</table>

Answer 1

It turns out this was an issue with IE failing when two different elements have the same ID. Apparently this breaks the .each function.
I had two tables
table.notes#a1 & table.inputs#a1
The .each function should have gone through each table but instead found neither.
jQuery also wouldn't run in ie with
jQuery('div.a1, div.a3, div.a4, div.a7','table.inputs#a'+tableId+' td:nth-child('+columnNum+')').each(function(){
alert(jQuery(this).attr('id'));
});
which it should have done, as I am them pointing directly to a specific table even if the id is not unique.
I'm using id's retrieved from the database for the id, and IE doesn't like id's that start with numbers, so I just added an 'a' to the beginning of the id.
However, it apparently doesn't like that either, so now I'm adding the first letter of the class and then the '1' or whatever the id number is.
This solves the issue.