Answer

By Your Horse

Show Jumping Rider and Trainer Carol Mailer says:

I think it’s significant that your horse jumped well when you first bought him, so you obviously bought a well-schooled horse. Is it possible that he is testing you out, and finding that you are not riding in a similar fashion to his previous rider? Or might there be something wrong with him?
Have you had his tack, back, teeth and soundness looked at? Speak to your vet. Explain the problem and ask if you could try a short course of painkillers like bute to be used purely as a diagnostic aid. If his enthusiasm improves, he may have been in some kind of pain, which has caused him to back off. Once you are sure he is okay, then you should look at your riding.
It seems as if there may be an inconsistency in your riding. Are you making an effort when you approach the fence? It’s difficult to diagnose without seeing you ride.
Speak to the previous rider to find out if they used spurs, and try using gridwork to help you.
Try not to count strides. If you are sensitive about your take-off point, it will only encourage overriding instead of maintaining your rhythm.
Things should improve as your riding becomes more consistent, but do check his comfort out first.