A few things will eventually help with this.
(1) If you are practicing acoustically, i.e. on an electric without the amp, then you may dig in too much to hear the pull off. It is a common issue with electric guitarists who practice w/o the amp. You need to hear what you want to develop the correct sense of touch.
(2) A common misconception is that the pulled off finger needs to "strike" the string for lack of a better term as hard as picking would. It has been commented that sometimes all you need to do is lift the finger rather than truly pull it off. Again, whether this works or not depends on your set up. If the guitar+amp combo is "hot" hammer on/pull off sequences become light tapping sequences. This is opposite to the situation on a classical guitar where one needs to really strike and pull the string with some force to get audible sound.
(3) Even though it is best to practice slowly and build up speed some techniques cannot really be executed slowly. Think of a cartwheel or somersault, you need to be in sync with gravity or you'll hurt yourself. If you dig in and pull slowly you'll hear the bend. As Tim points out, if you get off the note quickly you will not hear the bend as much. The 45 degree point may be more of a personal taste. You'll need to search for your own sweet spot where you feel comfortable with the technique and get the sound you want.
Again, it is crucial that you are playing with the setup and sound you would normally have to get a valid feedback between your sense of touch and expected sound.