ok, this is crazy but.... Do you get BBC america where you are? There is this hilarious show on called 'How Clean in your house" and they have ALOT of really good tips on it.
You may want to remove the shower head itself and look inside to see if sediment is stopping it up or maybe a washer is enlarged and has covered part of the opening. if not then: this is what they would start with.... if it isnt a brand new shower head, I would suggest getting a baggie large enough to cover the shower head and put plain white vinegar intot he baggie so that it covers the wholes in the shower head and let it set for about 30-60 minutes and then remove and run the water through it... and hopefully it will be fixed.
Let us know how it goes!
:2cents: