Of the bajillion times I've seen or heard of this problem, it's due to the low flow design of the shower head 99% of the time. Very often, if you unscrew the shower head from the chrome elbow sticking out of the wall, you will see a removable washer. That's a flow restrictor, looks like a normal washer with a small hole for the water to get through. Pull it out and it's all good. Often they're an add on, pull them and chuck 'em, but if they're neccissary to seal the head to the elbow, you'll need another washer that isn't a flow restrictor.
In the case of a "telephone shower" head, these are located at the shower head end of the hose.