There are a few things that can cause this:
-weak HP seat spring
-Seat has been sanded too many times (deprecated due to Bryan making seats)
-Using a new, thin first stage filter without a washer (I've done this)
-Levers set too high
Work your way through those and keep us posted on what you find

The two most common, in my opinion, are the lever height and using a thin filter with no washer.
The impossible missions are the only ones which succeed. -JYC