My pony custard is an unenthusiastic as it gets. He is very lazy, although in fairness to him, about 20x more forward than he was when I first got him! He gets a LOT of hardfeed (1 scoop of 14% high performance mix, 1 scoop of high energy apple chaff and 1 scoop of barley morning AND night!)
He also gets plenty of days off, sometimes 2 or 3 per week, I don't jump that often either.
I ride with a whip, but he HATES spurs, and as I can't use them in shows, I don't use them in case he relies on them to go forward. Sometimes I use two dressage whips, and I reward him with breaks when he's going well. The odd time we can achieve an extended trot, and 90% of the time he goes on the correct outline (also a HUGE improvement from 8 months ago)
I'm sure I haven't mentioned some things here that I've tried, but has anybody got any ideas?