This plastic filter you refer probably is the retainer that stops the check ball in the probe from blocking the flow. If there was no check ball present you do not need to be concerned with it sticking. Sometimes the ball is removed because it sticks (due to lack of cleaning). As Topgun noted, shake the coupler and you should here it rattle. If this is not the issue we may have to start at square one.
What is you liquid temp., what brand of beer, what elevation do you reside, etc. Please advise.
Scott Zuhse, Instructor
Micro Matic Dispense Institute