are you sure you use the proper injection techniques?
With this happening, I'm certain that I am not.
The confusion, for me, is that the first couple of injections were so easy. No pain; I felt nothing at all except (this will sound odd) a slight drag pulling out the needle. What I'm doing different now, I'm not so sure. The only thing I've noticed is that, rather than become more confident each time, I have grown increasingly nervous. So I insert the needle exceptionally slowly. My guess is that, as I go so slowly, I unintentionally move my leg. HOWEVER, I tried the ventrogluteal seated as well. Same thing.
I suppose, because I can do a dorsogluteal without this problem, I should just continue to do them. I'm disappointed because the ventrogluteal ones started out so effortless and really without any sensation of injection at all.