Profile Manager unable to push settings out

Hi,

I've recently purchased a Mac Mini (Mid 2011) for my workplace and set it up to be our MacServer using Profile Manager to push out settings to our iMac clients and iPads. I'm able to push settings out to my client I set up during the testing phase of our deployment and everything seemed to work fine. However now I have added the other 19 iMacs to the Profile Manager the Push Settings either stay 'in progress', 'pending' and eventually fail to all the new clients.

Here's the spec of our MacMini:

OS X Yosemite Version 10.10.6

Mac Mini (Mid 2011)

Processor 2.7 GHz Intel Core i7

Memory AMD Radeon HD 6630M 256 MB

Working in a school I have made all the following changes to our Firewall Ports (OS X Server: Ports used by Profile Manager - Apple Support)) and my Broadband Provider assures me all changes have taken place.

I would really appreciate any help with this issue.

Thanks

Ryan