Normally if it is only 1 or two zones operating with low pressure, I start looking for a leak. This may or may not be in the pipe itself, it could be a broken riser (the threaded nipple that connects the heads to the pipe), or a broken coupling etc.
Turn on the zone manually, by either using the bleed lever if equipped, or by loosening the valve solenoid about a 1/2 turn.
Start walking around from head to head wiggling each head to look for water coming from places it is not suppose to be. Maybe let it run a while to see if you can get a leak to come to the surface. Watch for lumps in your lawn that were not there before. If you have good turf, it can expand like a water balloon!
There are other less common problems that it could be, a pinched line from roots, clogged line from debris, or a faulty valve.
Do the suspect zones have more heads than the rest?