I have 4 wall mounted shower heads that form a square pattern. When I first moved into the house 4.5 years ago the top 2 heads had strong flow and the bottom left was weak and the bottom right was medium strength. Over the 4.5 years the flow in the bottom left has completely stopped and the flow in the bottom right has been reduced to very week (almost a trickle). (As an aside, one of my kids pulled off an broke the bottom left shower head.) My house has a water softener that's always worked well.
I have a theory. The calcium and lime that makes it through the water softener has been building up and blocking these heads. On the head that's come off I can see a wire mesh in there which I assume it to catch sediment and prevent it from blocking the small holes on the shower head. I think that the calcium buildup in the pipes has been falling towards the bottom 2 shower heads that are probably on an elbow and that's why the bottom 2 have stopped working and the top 2 are still okay.
How do I go about repairing or replacing this to restore the flow? Do I need to unscrew the stem from the wall and if so are there any precautions I should take before doing so?