I certainly don't know.
a wooden nickle fwiw though;
If you were here I'd say 'Maybe' you need more well points. Have to do some pi.

Here a 1.5hp shallow well pump typically gets 6 - 1.25" x 3' sand points. They say do the math on the pump flow chart based on 3 gpm per foot of 1.25" point to keep it from cavitating on the way up to the pump. Â
No quick bends or fittings to introduce turbulence near the pump intake for maybe at least a foot,, there's some rule of thumb math somewhere for minimum length of intake header so water turbidity smooths out before entering the pump.

Did you develop the well after installing it? That involves forcing water back down the well through the point screen using another source of water for a minute or so, and then running the pump - repeat many times until the water runs clear. That process, while a pain in the a$$, clears out all the small, clogging particle around the well point leaving larger particles that don't slow down the water as much. I'm not sure a centrigul irrigation pump is appropriate for a shallow well application either, but I'll let a pump expert provide an opinion. Any out there?

When you talk about pressure, how are you reading this. A 11/2hp centifugal lifting 21ft to the pump just about maxes that pump out on lift. Is there any way you can place the pump down at the well to make the lift at only 12 ft. If your running the pump at open discharge (not plumbed in) you probably wont notice much pressure as thos e pumps are built for volume and not pressure. Remember that you must design your sprinkler system around your water supply (gpm)or else you may be disappointed in your systems preformance. Email me if you need more help.