First off bleed the system to remove air.
If this doesn't work check for air leaks in the feed pipes
to the pump, you won't always see water leaking out, but if it's
slight it can draw air into the pipe.
It's also possible that a scum has formed on the filter and is restricting
intake.
If these don't work you're then looking at stripping the pump, as scale and
other foreign bodies can hold one of the poppet valves open.
How old is the pump and is it a lot noisier than it used to be, as wear on the
eccentric cam bearing can also give these symptoms.