Cisco 1720 Router Problem

I have a Frame Relay circuit with two drops (we’ll call them A & B). A Cisco 1720 router at each end. There are times that I am unable to communicate across the connection. I am able to ping routers A and B from the B side but cannot ping A or B from the A side. It becomes necessary to power off both routers to re-establish communications. Any ideas on what could cause this problem?