There were a number of problems with older versions not properly matching up the invoice in the BKAPINVT file and the payments in BKAPCHKH which is how the aging controls what is included.  You (or somebody in tech support) will need to go into Maintain Database and look at the affected records to determine what is going on.  Purging Vouchers will have no effect; it is the Aging and payment files that are the problem, not the voucher file.