Been chasing a leak

For years I have been chasing a leak. I have had two companies that can't find it.

After a few years it is worse. I seem to have narrowed it down to an interesting phenomenon. I have an in ground with an attached spa. I have recently noticed the spa losing water. The strange part is, it looses water when I have the pool pump running and the cleaner on. When I run the spa, spa jets, or even the pool skimmer with the cleaner shut off I don't lose water.

I would like some ideas before calling another company.

Thanks for any support.