Profiles stuck on "pending" Profile Manager

Using profile manager, el cap server 5.

Top level staff and student groups, containing settings. These profiles I belive get pulled down from the server and then stored locally on the machine of which they login too.

\library\managed preferences\USERNAME

The quirk I am having is that sometimes these profiles push and download without problems, and sometimes they get stuck on "pending" according to the profile manager web interface (see screenshot).

I cant see anything in the logs, so I must be looking in the wrong place.

Any thoughts?

thanks