VMware - Snapshot error encountered (156)

I have a VMware policy configured using annotation query to select clients.

Yesterday one of the clients failed with a status code of 156.

Last night the same client completed successfully.

I noticed in the detailed status, I have error's relating to resource.

23/09/2014 18:00:52 - Info nbrb(pid=2888) Limit has been reached for the logical resource pd-dc2-nbumst.VMware.snapshot.vCenter.pd-dc2-vcenter.pdports.co.uk 

What do I need to check/change to stop this from re-occuring?

Thanks,