Start by removing the rain-head and see what your throughput is.
If that's okay - you might find there is a build up of scum/scale inside the head.
Try the shower on full-cold - any flow?
Try the shower on full-hot - any flow?
The above two tests could lead you to a filter being blocked in either inlet.
Not sure if the mixer unit itself could provide a blockage - but if accessible - have a look.