We have had a number of reports of this but we can't duplicate it or identify the cause.  I will post this also to a new section called "Bug Fix Challenge"
 
6 months of free phone support to the customer that identifies the specifics that cause it to happen so we can fix it.  
 
Here is what we know:
 
RANDOMLY (as far as we can tell, but as programmers, we know it is not really random)
1.  This seems to be Evo only, not Classic but I am basing this only on the users reporting the problem.
2.  Invoice is entered in AP-C with no errors; all appears as normal.
3.  GL batch is out of balance; the  Credit posting to AP is missing
4.  Invoice is missing from the aging
5.  PO is properly saved as Invoiced and (as applicable) CLosed.
 
Most of the users that have seen this see it only once or twice per week with no obvious common thread so Carol, if you are getting it all the time, you may be a prime candidate to help us solve it.
 
You can either delete the entry in GL-P and re-enter the invoice in AP posting to PO/RNI or create the AP enty in GL-P and then enter the AP-B Voucher as Beginning Balance type but that it the short term fix for the symptom, not the cure.