I am experiencing the same problem. I first thought that maybe my dhcp lease was to long and that I was running out of IP's, but that is not the case. I've logged onto the vm through vcenter and tried to do a release and renew, but it still wont pickup an IP address. If I remove the problem vm trought the connection server then they start working again. I'm thinking maybe it's the network adapter. My VM's are set for flexible. I think I'll try one of the other choices.
I solved my problem but it is sort of specific to my setup. The number of ports in the vSwitch I was deploying the VM's on was 120, because of which only 120 VM's had adapters. I increased the number of ports to 504, restarted the VM's and now all of them have adapters and IP's from the DHCP server.
Glad you were able to resolve your problem. I fixed mine too. It turned out one of my host nics was in the wrong port on my switch so the vm's couldn't hit the dhcp server.