Skimmer Line sucking in air, pump losing prime

[gaelpride]

I have a 25,000 gal pool with a main drain and a skimmer drain for floating stuff. I'm somewhat new to pool ownership so forgive me if I call it by the wrong name or my comments seem obvious. About 2 months ago the pump lost it's prime and I discovered it was because my skimmer line was not pulling water anymore. The main drain line works fine. I shut the valve to the skimmer line wan was able to reestablish prime using just the main drain. When I open the valve to the skimmer, however, the pump immediately loses prime.

I immediately thought "clog," and had a pool company come blow my skimmer line to clear it. They told me there was no clog, and that I must have a broken line somewhere that's pulling in air, preventing the water from circulating. I called a leak detection company and they said they wouldn't be able to find it since it's sucking in air, not leaking water (looking for the "wet spot"). They said almost all situations like this are above ground and gave me a trick to try, which didn't work. It occurred to me that the issue must be above ground because even if I had a crushed pipe in my yard, it would effectively be airtight because of the soil compaction around it, right? So then water would be leaking and we'd be looking for the wet spot in the yard. Is my thinking on the right track? I did notice that the ball valve on the skimmer line turns around and around, rather than stopping after a 1/4 turn like the main drain. But it seems to effectively shut off the skimmer line so the pump is able to maintain prime from the main drain, so it's gotta be working, right?

Bottom line, can anyone give me some suggestions on the possible issue and how to fix it? I've decided I'm going to replace the skimmer ball valve anyway, but any other suggestions short of digging up my entire yard to see if there's a line break? My main drain is working for now, but it's struggling to keep the pool as clean as I'd like it.

[Denniswiseman]

The fact that the pump operates correctly when the skimmer line is closed proves the above ground is working OK as is your main drain
The fault lies within the skimmer line. Is the water level 1/2 to 2/3 up the skimmer opening, is the skimmer door jammed closed
When you are sucking in air, there is no leak but when you stop it will leak. Can you pressurise the skimmer line to see if there is a leak using expanding rubber bungs
Compacted soil is not air tight and it could be in sand