Unable to pick-up gravity leaks with the hydrophone. XLT30 H Series.

The big foot is great! No problems.

I am able to use the hydrophone when pipes are under pressure to pinpoint which therapy jet is leaking.

Example Pool is leaking around Main Drain lip next to Marcite. Hydrophone (XLT30) can't pick it up....I'll use my partners XLT16 and it is screaming.

I've made several adjustments but with no success.

Is anybody else having this problem?