I'm brand new to AR's but I was still able to bump on my first round of trying. Not my very first try, but my first few mags of attempts. So I can't really give too much advice, but I can cover a problem others may not think of.
First off, I couldn't bump from the shoulder. Tried once without any real concentration on my technique, so I'll try later. Then I tried on the hip and got it and had too much fun to bother going back to the shoulder. However, from the hip I basically locked my trigger finger in place by pushing it into my side. Didn't try it myself, but you might even be able to loop your finger around a belt loop. Then just grab the rifle and pull forward.
Second, the other problem I had was with mags. I tried with 20rd metal mags and bumped it all the way through. Then grabbed my 30rd Thermold(plastic mag) and couldn't get it to bump. Thought it was the different ammo I had, but then went back to my metal mags and success once again. Both the metal and thermold mags are brand new, have no semi-auto feeding problems, but the thermolds wouldn't work. Maybe they need to be broken in or maybe just no good for bumping. So if you have different mags, it might be worth a try to check them all out for bumping. And since it seems it's easier from the hip, I'd check them from the hip. Just lock your trigger finger in one point.