Only being able to get down to the semi-tone bends can be for a couple of reasons.
1. You just don’t quite have the right technique or feel yet.. and yep that is just a practice thing.
2. You are trying to hard (tense) and actually chocking the air supply, which makes deeper bending hard. In this case, just relax, take a few deep breaths and see how you go.
Relaxation really is the key to good playing!! As for moving on through the course, by all means move ahead but you really can’t just skip those bends. You definitely need them to play the blues scale and much more.. so keep them in your practice everyday until it clicks!