It sounds like it needs a good look at, i would say the main drain back up to the toilet needs looking at. Once this has been confirmed then shower, a snake tool or plunger can be used to help dislogde.
showers often block up from hair, grease etc. If there is access underneath the shower then the trap can be removed and cleaned,
failing these things drain acid will free the shower 100% please read this carefully;
drain acid is very dangerous on the skin or being inhaled.
drain acid will damage the tray on contact ( it has to be put in in a certain way)
i advise you get someone who has experience with this substance if you have never used it before.
if you need more advice please say