It sounds like you have worked in a bad habit. Try to slow it right down and just do one pull-off. Can you do one really slowly without hitting the other strings? Maybe try and just lift your finger off at first and then gradually work in the pull-off.

Also, I find by pushing the finger on the string that you are going to pull-off to very slightly in the opposite direction to the pull-off helps to increase the loudness of the pull-off and increases your control. It's sort of like push-up/pull-down at the same time. But watch your pitch you don't want to bend the string.

Hope that helps.