The best way to deep clean a shower is to firstly remove the shower head from the wall for better access to it.
Then, soak it in a solution consisting of a half and half mixture of white vinegar and water, contained within a sealed plastic bag or a basin.
The vinegar will help to loosen any particles of residue, dirt or grime that are contained on the surface, helping to eliminate the chance of clogging occurring and curtailing water flow.