Agree basically with the above(except I rarely find need for spurs & crop
). If your horse is not underweight, then the problem is not likely to be fixed with more calories... although too many calories is a common problem that causes laziness!
I think whole oats are one of the safer grains for horses & apparently one of the few grains that can be fed whole & digested, but think there's generally better alternatives anyway. While racehorses may need a high octane boost, I don't think this kind of 'sugar rush' effect is good for horses generally... or their riders!
Other possibilities besides training; could be a nutritional deficiency or a physical problem. Its common for people to think a horse is lazy when they're a bit sore on their feet or have a bad back or some such.