Hello Jeff, I hope the day finds you well!
In regards to the timer, you're right that in most cases it won't make a difference, but there is a small amount of pressure loss encountered when going through a timer or valve channel. While it usually doesn't make a difference, it could on a very low pressure system already on the cusp of having insufficient pressure.
I also wanted to note that you are correct: the hight of the water source does also influence pressure. Pressure is generated at a rate of 0.433 PSI for every 1' of elevation change. The height the container is off the ground and the height of the water inside do create pressure from gravity (though of course the height of the water in the container will go down as the water is used).
If the level ones start working again when the timer is removed, good chance that small pressure loss is the culprit. While off, it might be worth checking inside to make sure there are no obstructions, just in case some debris or foreign matter made its way in there.
Have you had any response outside this forum? I just installed a new system and have a similar problem. I have a gravity system with about 8 inches of water in the tanks (2). The tanks are about 16" off the ground and most of the drippers are below the tanks output. One section is nearly level with the output. I hooked everything up and it worked and then I installed the timer. Everything except those on level with the output work. I'm going to take the timer out and test again and then check/clean the drippers. I don't know why the timer would effect anything. I found a water pressure calculator on line that shows me the pressure based on the depth of the water. I thought height of the source wound influence pressure as well. IDK but from the calculator. the difference between the starting level and when I discovered the issue is about 1.5" or a loss of .054 psi. Beginning psi - .2702 ending - .2162. Seems pretty negligible but what do I know? Jeff Mattingly
We saw your email come through at the exact moment we were replying to this forum post. We'll get a reply over to you promptly to troubleshoot this one, it is definitely a curious one since the emitters work on start-up but do not continue doing so.
We look forward to assisting and will be in touch very soon :)