error backing up virtual machines

Hello

We are trying to back up 2 x windows 2003 server VMs. 

However, we receive an error :-

The job failed with the following error: Unable to create a snapshot of the virtual machine. The virtual machine may be too busy to quiesce to take the snapshot.

Previously these 2 Servers were Physical and were converted to Virtual Machines

Can anybody assist please

Thanks

Lois