lack of cooling water flow

Haven't been as active as usual in this forum, but have been meaning to post a question about an issue we experienced towards the end of last year.

Basically, the engine at startup would not circulate water through the cooling system.  Nothing appears out of the ordinary.  I have a small inline filter installed just before the muffler that I installed at least 5 years ago.  Right now our solution to the problem is to unscrew the cap of the inline filter which somehow gets the water flowing and then we screw it back on.  I cannot find any blockages in the system.  I also tried tightening all the hose clamps (all connections are double clamped).

Anyone have any thoughts as to the culprit?  

Writing this from a 6 year old memory......When I first purchased my Laser, I had a rust obstruction in the metal connection point where the small rubber hose connects to the thermostat on the starboard side of the motor.  I was getting partial water flow (didn't realize that at the time) and a consistent, slow rise to overheating.  I pulled the hose off, stuck my marlin spike in the connection point and water with rust debris poured out instantly. Cant remember which end was clogged so try both.   Never had a problem again.  I don't think water was flowing freely through the thermostat, which likely led to it not opening in a timely manner.  Worth checking...Two hose clamps and a little wrestling match in a tight space if I recall correctly.

  Ron