Make sure the unit is updated to the latest firmware release or higher.
See if the box is coexisting with vmpls, dns, and other VPN's.
Check the latency (Enable the 48 Hour Intensive Latency Test located under Diagnostics> Network Health > and scroll down to the bottom.)
VPN's use timing to authenticate and if the latency is tremendously bad you won't be able to generate a key because the times will not / do not match.