What could be causing our shower fixture to not work properly?
It sounds like when you serviced your boiler that some how or another some debris got into your water lines. It could actually just be some buildup that was already in the pipes that got broken free when the other work was going on. In any case this particulate has gotten into the inner workings of your shower faucet. 

You should be able to shut off the water supply and then disassemble the shower faucet so that you can get it cleaned out inside. There is a possibility that the rubber seals inside the faucet got torn and thus would need replacement as well. It is hard to offer specific advice on how to disassemble the faucet without knowing what make and model that it is. My experience is that most of them become pretty obvious how to take them apart once the knob and face plate are removed.