Paccar fuel filter doesn't stay full

[aaronb]

I have a 2013 Peterbilt 389 with an ISX15 cm2250. It has the PACCAR fuel water separator on the frame. The fuel level in the housing fluctuates. Awhile back I removed the fuel line on the inlet side of the filter to check for a blockage there. No fuel at all ran out of the line. I haven't had any issues with hard starting or cutting out. Sometimes the engine doesn't feel as smooth as others. Any advice what the problem might be? Thanks.

I added a couple pictures showing how low it is at times.