The answer to 'why is my supply line low water pressure' is that there are several common causes, including a partially clogged supply line, a problem with the main water valve, or low water pressure in your home's overall plumbing system. Symptoms may include weak water flow, low water pressure at multiple fixtures, or water pressure that fluctuates.
Some potential solutions include checking the supply line for any blockages or kinks, cleaning the supply line filter, or adjusting the main water valve. If the issue is with the overall plumbing system, you may need to call a professional plumber to inspect the pipes and identify the root cause.