Another thing to check is actually make sure that it is seated all the way down. Its hard to get it pushed all the way in when the push angle is not centered over the hole and the tube is curved (it hard to get a good straight push).

When I was younger I would push them in but it would stop at the oring and I though I had it seated, I had perpetually leaky trans. One time when I was trying to get it to stop I got the right angle on it and it slipped further in. Problem solved!

Just something to check.