2008 R2 host loses licenses intermittently

I have a 2008R2 RDS Host. it is a VMWARE VM. I have specified a license server and when I run Licensing diagnostics, the host lists the license server, shows the correct number of available licenses. Every so often (1-3 weeks) the host shows no license server, and people can no longer log in. I run diagnostics and it communicates with the license server and everything is fine again.

Any ideas?

~J