Shower diverter valve - normal pressure on 'tap' setting, but very low

Recently moved into a house with some oddities. There is a bathtub with jacuzzi jets which seems to be working ok. There was no shower facility however so I bought a diverter valve so I could hook up a shower hose to the existing tap The water pressure when I set the valve to 'tap' mode, e.g. just let the water run to fill up the tub is fine. However when I switch it to the shower hose, the pressure completely drops to a trickle. I've tried 2 different shower heads that say they boost water pressure and 2 different shower hoses that say the same. The result doesn't change unfortunately - I thought it might be to do with the valve but it's fine when it is in tap mode Any thoughts? Thanks