I replaced my LP regulator this week. The new regulator is supposed to automatically switch, however it didn't. I left tanks open, but when the first tank ran out, no switcheroo. I noticed that I also couldn't get the 2nd tank top flow through even after annually flipping the valve. I ended up simply moving the hose f ft om the empty tank to the full tank for use. I'm wondering if the problem is with the line that was originally the 2nd tank or if the valve has to start from a specific side first. The instructions with the regulatory didn't indicate.
No high pressure regulator valve. Just the pig tail direct to the 2 stage regulator. I'm willing to bet it is just the pigtail. I am glad I replaced the regulator as well though since the one on it was the original from 20 yrs ago
The pigtail does have a ball valve inside to restrict flow until the line pressure equalizes to guard against an open line. And it can fail. Opening the valve too fast can cause the restricted flow, but the valve is supposed to let a small amount of flow bypass it until the line equalizes and then release the valve. But the pigtail is relatively easy and fairly inexpensive to replace to see if that is the problem.
Do you have a small high pressure regulator valve between that tank and the switch over regulator valve? If so, it is possible there is a problem with that valve. Another easy and inexpensive thing to replace.
Sounds more like problem in the second tank pigtail, or if you opened the tank valve too quickly you could have shut it down. A simple cure for that is just to turn the valve off then turn on slowly.