Don't know what a power shower is, but, the valve or valves feeding the the shower head have issues. Washers and or seats need replacement, if possible, you may even need total replacement of valve/valves. Find another the plumber, this is a remedial problem that your plumber should have picked up on.
I am not a plumber.

Did your plumber tell you what he was doing would cure the problem? Yes? then have him back to finish the job before paying him. No? Get another plumber and have him fix the problem instead of throwing parts at it,taking your money and leaving the problem for you to deal with.