Also make sure you don't have too much position. If the quarters are leading it's very difficult for the horse to maintain impulsion. If the shoulder is leading and the position is correct it'll be easier to maintain impulsion.
there are usually two reasons why the horse loses impulsion in a half pass: one, the horse is not strong or balanced enough to sustain the same rhythm and impulsion
two, the rider blocks the horse's movement with his seat
depending on which one is the reason (or both...) there's different ways to correct it.