More ideas of a completely uneducated nature:

Are you sure it's the right model of cartridge?

Is it possible the builder plumbed the filter backwards? Maybe the flow going out-to-in works just fine with no cartridge in, but not so well with the cartridge in.

Is it possible that there is some type of internal support ring that's missing from the filter? That is to say, when you put the cartridge in, it rests on the floor of the filter housing, covering up the outlet instead of being supported up above the outlet?

I wonder if the downstream check valve may be sticking. Without any cartridge in, the pump can force it open. With the cartridge in, the pump output is restricted to a point below the threshold required to overcome the sticking.