Keep in mind that the router will only react to FECN/BECN values if you are running Frame-Relay Traffic Shaping.
By default, the router does nothing at all, other than display the values for you.
Nevertheless, wybnormal's point was a good one: many things can cause dropped frames. Sometimes it can even be intentional. Example:
A client I support has a large frame-based network. They have a number of internal critical business apps as well as Internet access. They did not want Internet traffic impacting their critical business apps.
At the time, some of the queueing strategies were a bit new, and this client is scared of the bleeding edge.

So we put in a second set of PVCs just for Internet access. In our network, PVCs by default have a feature called Graceful Discard enabled, which means that red frames aren't dropped unless there is congestion. On their Internet PVCs, however, we disabled graceful discard, so the switches dropped red frames regardless, thus capping bandwidth at CIR (actually CIR+Be).
They're no longer scared of fancy-shmancy queueing schemes, however, so we'll be eliminating the second set of PVCs and implementing CBWFQ.
There go my evenings this summer.
--chris