Fax Integrator Issue's

We are using fax integrator version 9.0.0.2 (and have been for awhile) but lately we are getting numerous complaints from vendors stating they did not receive a fax PO from our system. When I look at the fax integrator and the outbox in the fax console it show the faxes were sent successfully. Also, PO66 shows they were faxed. This is occurring with multiple vendors. Its not occurring every time either, sometimes they say they received our fax and sometimes not.just curious if anyone else has experienced the same issue? Thanks!