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Remove the showerhead and check to make sure debris isn't blocking the flow of water
Once the showerhead is cleared of anything blocking the water flow, replace the Teflon tape. Wrap it clockwise around the nipple, overlapping it two to three times -- this helps seal the threads and prevent leaking.
Replace the showerhead and check the water pressure.
If the water flow is still weak, remove the showerhead again and double-check to make sure that water is flowing out of the nipple. If the flow is good, that means there's more debris down inside the existing showerhead. Replace with a new showerhead and that should solve the problem.