heads are not popping up, need help

I tried to turn on the sprinkler yesterday, and noticed at least two of the heads are not popping up. They are in the same zone. I kind of remember where are these two heads are. They are covered by the soil and grass and not able to pop up. I know I may need to use screwdriver to poking around those area where I think they are. But that may damage quite a lot of grass until I can find them.
I'm not sure this problem is normal or not? What is the best practice to find the sprinkle heads covered by grass?

Thanks for reading my questions.

Buried Heads

If that zone is on, you should be able to see at least a trickle of water coming from those buried heads. Once you find them, you need to clear out all the grass and dirt that is blocking them from coming up properly. If you don't see any water, you can poke around with a screwdriver or refer to the map of your irrigation system if you have one.

Poking around with a screwdriver will not hurt your grass any.

Found two, going after another one

Thanks Verdeboy. I found the two buried heads.
Now, I'm drawing map for my sprinkler. I believe I'm missing another head in another zone which I don't think I saw it last year. If I'm right, it was buried for at least two years. It's harder to find this one since there is no trickle water at all. I poked a few spot, couldn't find it. I'll try harder next weekend.

If you can't find it by poking, you can try digging around the closest head to the missing one. You can then see which direction the line is going, and it should lead to the missing head.

Leave the zone on for a while.
You should see a puddle develop in the vicinity.