My instructors always use lots of transitions to help build up impulsion, and I have to make sure that I always keep my leg on when I'm riding, even if I'm not squeezing. Yesterday I got to ride a really good horse (the stable owner used to showjump with him) and I really felt the difference when he started listening to me. You should feel a response the moment you give an aid, even a squeeze, and if you don't, step up to the harder aid immediately. All lesson yesterday I was told I wasn't being firm enough - but I hadn't ridden in two weeks, so I was tired and my muscles just weren't going xD That's my excuse, anyway, and I'm sticking to it.
Try doing lots of transitions at unexpected points, progressive and direct, and be really firm with each one. Also try riding more interesting things - don't just do circles at each end and go large round the arena. Come down on the three-quarter lines, do figure-eights and serpentines, and circle from B and E. Keep up the transition work all the way through and reward your horse for good transitions. I always find that after I've done that for a bit the horse becomes much more responsive all-round.
If you believe everything you read, better not read.