Sometimes if the poppet is not seated right inside the post it will not allow the disconnect to depress it completely, which will prevent the disconnect from seating all the way. I'd remove the post and separate the poppet from the post and then re-assemble. You say the poppets don't come out of the posts on this keg, but they should. Might take some persuasion, but they will come out. When I get a stuck poppet, a good firm press on the top of the poppet with a smallish phillips screwdriver with the post sitting on a hard surface always does the trick.