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My house came with about 10-12 sprinkler heads connected to one zone
for the front lawn. It is close to the water supply (at driveway),
even though most people would say there is too many heads.. It used to
work somewhat reasonbly. I only noticed one or two heads not popping
up, couple months ago.

Today I noticed about 4-5 of the heads that are further down stream,
are not popping up. I removed one of these heads and let the water
run, and the pressure is weak on those head locations. It seems like
the pressure suddenly dropped somewhere in the system, causing the
heads downstream to no pop up.

The main question I have is that when I turn the knob of the valve,
after a turn or so, the water flow does not appear to increase, is this
normal, maybe the valve is damaged and I'm not getting full flow. (I
have been turning the knob a bit)

Or I might have a crack somewhere, I do not know the routing of the
system so I might have to dig around a bit.
I also wondered if there was a rock/debris or something stuck in the
system. Which I might need to unearth the pipes to check
Or perhaps the water company doesn't have enough pressure lately?

Which should I look into first?
I wanted to do a complete re-installation using bigger pipes eventually
and less heads, but if I fix this I can wait until next year.

TIA

I have a timer operated sprinkler system parts of which are 40 years old.
The older parts used that plastic pipe that came in rolls -- ABS?, -- not
the modern polyethylene. I have frequent failures that are characterized by
the gradually diminishing flow that you observed. These are almost always
the result of a pipe which has split to allow gradually increasing leakage
and loss of pressure at the heads..

Sometimes the solenoid valves fail, but that usually results in a sudden
reduction of the flow to the sprinkler heads.

The pipe leaks are sometimes hard to find. In my case, it helps that I know
the routing of the pipes. It could be tough otherwise. As others have
said, you first look for overly wet spots above the failed pipe. --- SJF