A couple things to check:
1. Disable ECM on the fax machine
2. Set the transmission speed lower.
Newer Fax machines are SG3 capable which mean they do not negotiate t.38. The use modem speeds for faster transmissions. ATA-187 are not the best solution for Faxing. I would use either a POTS line or a different voice gateway such as a VG302 where you can use H.323 or SIP.
Jason
Please rate if this helps. Thanks!