The first thing I would do is go to GL>Main>Daily Transaction Register and run it.  If any Daily Transaction Registers from other modules were missed, this one will pick them up and post to the GL to make sure you have everything.

The month that doesn't balance is a previous month.  So the Daily Trans Reg have been run many times afterwards. 

The bank recs for following months that I've done afterwards all balance other than that same amount from the month I'm having a problem with. 

What would your next step be?

"So the Daily Trans Reg have been run many times afterward".  From the GL menu itself?  If they were ONLY run from the module themselves, and not from the GL>Main menu, if one was missed, it would not be picked up.  In other words, if you run the  Cash Receipts Journal and update it, and then when it asks you to run the Daily Transaction Journal and you say No, the next time you run the Cash Receipts Journal and Daily Transaction Journal from AR, it will not include the prior one.  I always recommend to my users to run the Daily Transaction Journal from the GL menu at period end, just in case. Maybe you already do, but your response wasn't clear, so I just wanted to make sure.

"So the Daily Trans Reg have been run many times afterward".  From the GL menu itself?  If they were ONLY run from the module themselves, and not from the GL>Main menu, if one was missed, it would not be picked up.  In other words, if you run the  Cash Receipts Journal and update it, and then when it asks you to run the Daily Transaction Journal and you say No, the next time you run the Cash Receipts Journal and Daily Transaction Journal from AR, it will not include the prior one.  I always recommend to my users to run the Daily Transaction Journal from the GL menu at period end, just in case. Maybe you already do, but your response wasn't clear, so I just wanted to make sure.

Yes, from the GL itself. I will double-check tomorrow just to verify but normally, anytime I post a JE or recurring journal, everything is always updated. 

Okay, I verified it. The file was empty.

What can I try next?

It could be something with how Sage recorded invoices or something to that effect. But I can't find a way to check information other than the GL Detail, which all checks out okay. I don't think it's a typical posting error. 

Again please note that if you post a normal JE in the GL it will NOT hit bank rec. Only Transaction Journal Entry will hit bank rec.

I'm doing a MANUAL bank rec. I'm not using the bank rec module. Some JEs in the GL do affect my manual bank rec. The GLs aren't the problem though. I think the issue has to do with AP or AR. I just don't have any way to verifying the info other than using the GL Detail Report. 

Are there other reports I could use to find the problem?

Everything posted to a bank code from AR / AP should be represented in the GL... with the only exception being if the DTR was not posted through.  Run the DTR from the GL module to make sure that is not the case.

If you have problems with a posting amount, looking at the PDF journal / register is often a good place to start troubleshooting.