I learned a couple of things early on about the bending process. A friend who played suggested that I whistle a high pitched note and then to slowly lower the pitch of the note to as low a note as I could, all the while paying attention to what the tongue was doing, and more importantly what the tip of the tongue was doing. You will notice that the tip of the tongue is initially, (when on the high note), up close behind the lower teeth. As you lower the pitch of the whistled note, it, the tip of the tongue, slowly moves away from the teeth, downwards and back towards the base of the tongue. Do this a few times all the while noticing what the tip of the tongue is doing.
When you bend a note on the harp, the tip of the tongue goes through the same motion and somewhere in that motion the tongue is in the right spot to produce a bent note. Now the problem is that the position of the tongue is critical, it must be in just the right spot to produce the bent note; and that position is different for each hole. What this means is that if your tongue is too far back in the mouth to begin with, you will never get the reed to bend. Or if you move the tongue backwards too quickly, you might move it right past the spot where the bend can occur. This is especially critical on the 6 draw bend where the movement of the tongue is so small.
You can test this out very easily on a hole where know how to produce the bent note. Start out with the tip of the tongue up close behind the lower teeth. Start to draw in and very slowly move the tongue down and back. Do not stop when the bent note has been produced, but continue to draw in air as before and continue moving the tongue backwards. The bent note will be lost and the pitch will rise back to its normal pitch.
So just be very conscious of where the tip of the tongue is and move very slowly. I hope this helps you out as it did me.