jv,

To answer a question like this I would have to know or look at your system. By altering your systems flow design you may or may have done things to affect the hydraulics of your system or it could be mechanical with your valves. It would be best to have a local irrigation specialist to come over and check your system. You have to tell me the pressure and flow rate of your system and the number and type of heads you have in each zone. I would return the system to its original condition and see if the problem goes away. If that happens, your alteration were too much friction and flow loss for your system. JMHO GL Aloha

That was a lot of good technical information JMHO.
However, maybe just try digging up that impact sprinkler you installed first.
Two, it might be broken or blow apart under ground at the swing joint, is what we call it. Unless you connected it directly on to a tee and it has no flexibility. Which will constantly keep breaking. (This will cause pressure loss).
Three, if you have spray heads and you put an impact head(sprinkler), then this will not work. they put out less water. *** Just let me know what you find...

For some reason I rotated the bleed knob (I think that's what it's called) on the valve and the sprinklers started working again both manually and via a timer. So far so good.

Thanks again.

Good Job! FYI - 1). Most residential homes have valves with two nobs on top. 2). Large nob is the flow control. Which will adjust how much water comes out valve and you, or someone could have by turning it all the way to the right, shut it off. 3). Small nob is the bleeder. Which simply put, turns on the valve manually.