There are several reasons why this could be happening. The most obvious is that to large of a mesh size is being used. You can refer to pg. 90 of our current catalog, which can be viewed at www.banjocorp.com, for standard meshes and data regarding them.
Another common reason is that the body o-ring on the strainer needs lubricated. Over time the lubrication will wear off causing the dry o-ring to be rubbed up against the dry plastic housing. This dry condition will not let the bowl screw up into the housing and seat the screen as deep as it needs to. We use a liquid silicone when we assemble the strainers at the factory. However a cheap and readily available alternative is to use canola oil.
Lubing the oring will also keep it from stretching out and causing a leak between the bowl and housing.