I've rarely known them to get completely stuck, although certainly
ones that have not been removed for years perhaps can sometimes be
stubborn. But in the end, with persistence, you can usually move them.

The key is to ensure that the ISS is still firmly secured to its
mounting point so that all the torque you're applying to the cover
does have a firm base to act against. It can be pretty tricky trying
to remove the rain gauge cover from an otherwise unmounted ISS.

But then it's 'just' a matter of applying enough torque to the cover,
eg with one palm on each side of the cover, trying different positions
around the cover if necessary. Sometimes if you look closely you might
be able to see which of the tab positions might appear the most
unyielding and then putting some extra downward pressure around that
point can help. With a really stubborn one then tapping the tabs (in
the right direction!) with eg a slim piece of kindling (ie a slightly
yielding bar/rod) and a rubber hammer might do the trick.

John Dann
www.weatherstations.co.uk