2 of the keys are sticking, because the plastic has warped with time or
temperature. Instead of internal dimension of 18.7mm the underside open
width is now about 18.3mm where it abuts the static pillar and rubs against
it. Any recognised safe method of locally heating or something that will
accurately realign the plastic and retain reformed shape? the throw on one
side is likely different to the other side so needs taking into account.