Diagnosis
This is almost always a clog in the system, either in the brush head's water channel, the handle's internal tube, or the reservoir's stem filter.
How to Fix It
Let's clear it step-by-step. 1) Remove the brush head. Turn on the flosser (just the water mode) and point the handle over the sink. If you get strong pressure now, the clog is in the old brush head. Soak it in hot water or replace it. 2) If pressure is still weak from the handle, the clog is upstream. 3) Unplug the base. Remove the water reservoir. Look inside the black rubber gasket in the base—there's a tiny mesh filter screen. Use a toothpick to gently pop it out. Rinse it under warm water to clear debris. 4) Run a cycle of lukewarm white vinegar through the system (without a brush head) to dissolve any mineral clogs in the handle's tubing.
If That Didn't Work
Check that you're using the 'Floss' or 'Floss+Brush' mode, not just 'Brush'. The brush-only mode sends very little water. Also, make sure the reservoir's pressure dial on the front is turned up to a higher setting (like 7 or 8).
Heads Up
Never use mouthwash or anything oily in the reservoir—it will create gummy clogs. For cleaning, only use water or a vinegar/water mix. And always run a quick water cycle after each use to clear the tubes.