"Pop-Up" Sprinkler - Half Up

my sprinkler system has been working fine. However, two of the sprinklers only pop up half way whenever it's activated. Does anyone know the cause? if you do, what is the fix?

Where are the culprits in relation to the valves and the other sprinklers? Is this an automatic or manual system? On mine, when the actuator went bad, there wasn't enough flow to get all the heads to pop-up.

Probably dirt in the shafts. Lift them up manually and clean them.

When you look at the physics invoved ( pounds of upward force applied against the stem) then failure to pop up is almost always due to low pressure in the line, either generalized, or low pressure at that head due to an underground leak, or a lot of leakage around the stem.

First rule in sprinklers....everything is cheap and easy, so don't lose any sleep trying to figure things out. You should have immediately just replaced the culprit heads, and then if problem persists, start looking for other leaks, or a valve not opened all the way.