Lower the pressure on the pump air regulator
Disengage the catalyst pump
Direct the hose into a bucket
Open the valve on the bottom of the filter
Slowly turn up the pump air pressure and verify that the resin flows freely from the drain hose

This will verify that the pump is working correctly.

Next:
Close the valve on the resin filter and remove the tip from the gun.
Leave the catalyst pump disengaged.
Pull the trigger on the gun and verify that resin flows freely as you turn up the pump air regulator.
The pump should speed up when you pull the trigger.

This will verify the hose and gun are OK. If so, the resin is probably cold and will have to be warmed up or you will need the most pressure you can get to spary the resin. Try turning up the pressure (up to 100 psi) to see if the resin will spray.

It is not desirable to spray at high pressures, it is much better to warm the resin to a minimum of 65 degrees (F).

The warmer the resin, the lower the pressure required to develop a spray pattern.