I had the same problem. I believe this is because the probes are larger than the standard 1/4 port. I remember measuring mine and finding it was larger.
I removed the plastic with a razor blade, sanded with 100 till it fit roughly, and then sanded with a high grit 300-400 to smooth it out. I have two and they seem to work fine.
Keg1: Belgian IPA
Keg2: Wee Heavy
Keg3: Vienna Lager
Secondary: Rye Wine, Gluten Free Saison
Aging: Bourbon Barrel Imperial Stout w/ Brett, 2009 Lambic, Kriek, Framboise, 2010 Lambic, 2011 Lambic