Hand full of devices unable to sync after adding a new CAS role

Hi All

We have added another Exchange 2010 server in DAG and have also additionally separated CAS from one single server to another VM.

Everything works well but only a hand full of devices are unable to Sync.

The devices are OK, I have checked the active sync and that works perfectly fine from the server end.

I have also tried added a different user on the same phone and it works, at the same time added that user on a different mobile device and that works too.

Has anyone come across this issue before.

Please help.

Regards

Sam