Loss of water pressure in shower?

I cleaned the shower head - left it to soak for a couple of hours, put it all back together and now it just comes out as a dribble.

It works if I lean it over the bath, so it looks like it's the water pressure. As soon as I put it back up high it comes out as a dribble.

Does anyone have any ideas? It's not a power shower its a tap and shower attachment all in one.

Many thanks in advance.