If the water pressure is very poor in your shower it may be due to lime scale build up in the head.  If you live in a hard water area you should clean your shower heads every few months.

To descale, put some white vinegar (or lemon juice) in a plastic bag and tie this over the head so that the  scale restricted holes can soak in the vinegar.  Leave a few hours and it’ll be like new.

Try this before you call the plumber for service.