If your shower struggles to produce water (especially hot water) and spits or splutters in use, the most common cause is an air lock. Air locks normally occur within the pipe-work of the hot supply and can develop over time or occur all of a sudden. You may find that the air lock only affects the shower and all your other appliances are fine - this would be because the shower should have its own dedicated supplies and the restriction has developed within this pipe-work.
There is a simple procedure you can try, using the shower to clear the air from the pipe work:
Remove the shower head from the end of the flexible hose or fixed arm,
Start the shower on the coldest setting,
Slowly turn the temperature dial towards hot,
If at any point the shower begins to spit or splutter then stop turning the dial and wait for the air to clear before continuing towards the hot,
You may have to repeat this process a few times to clear the air entirely but afterwards the hot and cold flow should be the same.
The idea of this exercise is to use the working supply to create suction down the pipe-work which will then siphon the air out.
If you find you have recurring air locks you may need to consult a plumber, or if the air will not clear and the flow continues to splutter then you may be experiencing cavitation, draw down or possibly an air leak.