If it was me, I would disassemble the faucet, removing stems and/or cartridges then flush out the faucet to see the volume coming out of the faucet body. Often times dirt and debris become lodged inside the faucet and block the passages. If have good pressure without the internal parts, then put the parts back inside and you have bad pressure, then there is something wrong with either the spout or the showerhead. Make sure you check the parts (especially cartrides) for debris inside the cartridges. If you have a pressure balanced faucet, sometimes that can go bad and you have to replace the pb cartridge.

Good Luck

Raymond VinZant Plumbing Prof.