For an existing shower, you can give the shower head a thorough clean, ridding it of any dirt or debris that might be halting a free flow of water from emerging.
Alternate options include replacing the shower head with a new model or installing a shower pump or pressurised unvented cylinder.
Or, you could go for a full-scale change by purchasing an electric shower or power shower with a guaranteed superior flow rate to your existing model.
The virtues of the latter components are discussed in more detail in our blog article, What Is A Power Shower?