Occasionally, due to a potential variety of factors (e.g. wet parts, heat, cleaning conditions), the valves can pop off the top of the lids while you're trying to assemble them.
Two quick fixes:
(1) If wet, dry off the valve and the bottle.
(2) Ensure a complete seal between the valve and bottle by running your finger along the edge.
Still having issues? Email or call - firstname.lastname@example.org, (509)985-8092.