There are several causes for low water pressure in your bathroom's faucet. Issues with water pressure usually have to do with the home's plumbing, but if you are experiencing low water flow only in your bathroom, the problem may have more to do with the faucet itself.
Blocked Faucet Cartridge
A common problem is with the cartridge in the bathroom faucet. Sediments and particles may block the holes in the cartridge, reducing water flow into the sink. This most is likely the problem when any one single faucet is experiencing low water flow. Sediments in your water build up inside the cartridge on a regular basis. To solve this issue remove the handle and unscrew the bonnet nut underneath it. Then take out the cartridge, a small disc-shaped object, and replace it with a brand new one made for your faucet.
Lower Water Flow Faucet Installed
Some faucets are designed to have lower water flow rates. This is a function of the cartridge inside the faucet. The lower flow faucet saves money because it uses less water. If your faucet was installed before you moved in and has always had low flow rates, this might be the reason. If you just installed the faucet yourself and the flow rate is low, check the manual to see if this is a low flow faucet. If the faucet used to have higher flow rates, the problem probably is a clogged up cartridge, as mentioned previously.
Temporary problems also may cause a lower water flow rate in the bathroom sink faucet. These includes a blocked aerator, which occurs when particulate matter becomes lodged in the mesh of the aerator screen, or a blockage in the pipes running to the bathroom. To fix a blocked aerator, unscrew the aerator from the spout, wash it out in clean water and then replace it or install a new one to see if this solves the problem.
About Plumbing Problems
Typically, homes with plumbing problems have low water pressure in all faucets, not just the bathroom. The problems range from sediment build-up in the pipes to more advanced issues, like a damaged water pump. When only the bathroom faucet has low flow, the problem must be just in the pipes in the bathroom. Check the valves under the sink and make sure they are all the way open. If not, turn them counterclockwise to open them. If they are open fully, turn off the water valves, disconnect the faucet hoses from the water pipe, inspect them for sediment build-up and clogs and clear them away. For more serious issues, the services of a plumber is highly recommended.