Calls dropped when transferring

We have a Fortinet 570, along with a number of Fortinet D71s. On occasion, when transferring a call, the call will drop, seemingly at random. There does not seem to be a pattern to this, but sometimes another D71 will already be engaged in a call when this occurs. Also, sometimes the transfer key (for a staff member) will be non-responsive, and the call will drop upon tapping the key. What are some possible explanations for this?

I can provide more details if needed.