yeah, its a conditioning issue. I used to have this problem when I trained all low rep sets, focusing on heavy weights.... my strength endurance sucked... once I started training again with 8-15 reps sets and lower weights, my body became better at it. Vanilla HST is interesting in that you do all different loads and rep ranges in a 6-8 weeks cycle, so your muscles have to get good at sets of 15, as well as sets of 5, or even negatives.