There's something hidden going on with training. It isn't just straight within 20 for tactics and within 40 for wrestling. Those are the general rules, however, certain mobs will train combat skills, while others will not, even though they fall within that range. It may have something to do with the parry skill that we have no way of knowing what it is.
So having said that, the best mob I have found for training dragons is the sphynx. Shadow elementals work well up to around 115 wrestling, but after that the sphynx seems to work much faster. I didn't pay that much attention when I tried training on swoops, but it does seem to me that I had the same experience you did, no gains.