The answer to 'why is my drain trap low water pressure' is that a low water level in the drain trap is usually caused by a blockage or leak in the plumbing system. The drain trap is designed to hold a small amount of water, creating a seal that prevents sewer gases from entering your home. If the water level is low, it can allow those unpleasant odors to come back up through the drain.
Some common causes of low water pressure in the drain trap include a clogged drain, a leak in the trap or nearby pipes, or a problem with the vent system that allows air to be drawn into the trap. This can happen over time as debris and gunk build up in the pipes, or if there is a crack or hole somewhere in the plumbing.