Shower mixer tap stuck

[porkchop]

My shower mixer unit has a chrome nob in the middle which diverts flow to shower or tap when pulled up or down. This nob has become stuck and very hard to operate.
Any suggestions of how I can fix this. Thanks.

[BlueToffee]

Have you tried WD40?

[tamborine]

try a waterproof lubricant, like oil or vaseline. Wrap a dishcloth on a hammer head & gently tap around the nob to break any limescale seal.