Firstly you need to check the water feed at the back of the shower is no kinked reducing the water flow. This can be a simple fix and is common when pushing the shower back in to position. If so straighten it out and test the water flow.

Second, isolate the water and remove the cold or hot water feed from the valve inlet. Place the open end in a bucket and turn the water back on and check water is coming out of the hose at a good flow rate. If so, re-attach to the valve.

Are you on a combination boiler or pumped system? If you are on a combination boiler the flow rate for the cold will be the mains pressure so ensure this is up to the desired level. If you have a pumped system then ensure the pump is kicking in, you will hear the pump working.

In 99% of all cases the issue will be a pressure supply problem and all can be checked out by your plumber.