Mira Zest....No pressure

Now I know the Mira Zest isnt the greatest shower in the world but I have a problem. The pressure has dropped right off and the shower doesnt make the same sort of "noise" !! anymore when I turn it on so I presume somethings not quite right. The temp is still ok and I havent noticed a drop in water pressure elswhere.
Any ideas what it might be ?
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