Are you pre-moistening the mix before you use it? It can be hard to get completely dry mix to soak up water in a pot and may take some time. Perhaps what you're doing when you use that probe to poke up through the holes is just allowing all the water to find a clear channel and instead of saturating the mix, it's just running through. Perlite shouldn't be a problem as many people use a layer of perlite at the bottom of the pot before adding their soil-less mix on top. That drains just fine.

Thanks annecelia
I am prewetting mix before placing in pot but maybe not enough; I will wet mix more and see what happens!