My stem isn't staying locked into my valve plunger.

The stem is "popping" out of place when I raise the lever.

The stem is designed to slot into the plunger and be rotated 90 degrees to "lock in" at a perpendicular orientation. The gauge should then be facing front. 

If the plunger slots are incorrectly oriented, preventing your plunger stem from locking in at the correct intersection, use either 1) your stem to pull/wiggle the plunger out of the brew head and then reinsert at the correct orientation or 2) the Valve Plunger Key to reorient while in place: Plunger slots should be at the 12 and 6 o'clock position. 

Note: It is normal to have to reorient the plunger in this way every so often, as it will incrementally rotate with use.