If you have a shower head that hasn’t been cleaned in a while, chances are you are not getting the water pressure you once had. Even if you have soft water, mineral deposits or rust from pipes, etc can build up in and behind your shower head.
Remove the shower head and empty out any larger particles. Then submerge the shower head and any parts into a container of vinegar. Some people suggest heating the vinegar first, so the process works faster and more complete. Leave for about 10 minutes.
Then simply replace your shower head and use as normal.