When it says you should get 15 GPM at a pumping depth of 10 ft it means when the suction pressure at the inlet of the pump is 10 ft (4.33 psi) below atmospheric pressure. Check the flow for 15 and 20 ft of lift.
You should find what the real lift is when all variables are considered.
First determine the height of the suction of the pump above the water in the well. The water draws down to 5 ft below grade when pumping. How far is the inlet of the pump ABOVE that grade, or above the water in the well when pumping?
In addition to that difference in elevation, you must add whatever pressure loss there is through the foot valve, and the friction loss in the pipe from the well to the pump. If there are any valves or elbows in that suction line, there will be more loss. There will be more loss if it is quite far from the well to the pump. You could measure the vacuum at the inlet of the well with a special gauge, or you could set up a manometer to measure it.
Another cause of poor performance in a jet pump is an air leak on the suction line. Polyethylene pipes with clamped joints are notorious for air leaks. PVC pipes are usually less susceptible to that problem. If there is such a joint on the inlet of your pump, you could try to seal it up with some kind of putty to see if it helps. That would not be permanent.
That exhausts my ideas for the moment.