Why isn't my Fluidmaster installation working correctly?
1 The float must not be near any obstructions..  Sometimes the valve body was rotated too near the rear of the tank, causing the float to graze and hangup.  

2 Another problem can be the refill hose is cut too short, causing the valve to bind.

3 Another issue is not extending the valve tube (it rotates, getting longer or shorter, affecting the tank level.  

4 Also check under the valve top, for bits of rust/pipe crud, which can cause very flaky operation.  Be sure to turn the water off for this one...