If it is a volume problem, try taking the shower head off and seeing if it has a flow restrictor in it as most of them do. Code says 2.5 gallons per minute is all they can do, but some older heads will do more than that. Make sure the flow restrictor is not plugged up as the holes are very small. There is also a possibility that the valve or the piping from the valve to the shower head is also restricted in some way. If you are getting good volume from the shower with the head off, it’s a restricted head. If not, call in the professionals.