I wouldnt be too worried, at least you are getting clears more often than not. Echo what other people have said. Also some horses are either naturally dangly and never lift above what will actually get them over the jump.
You say he took a hold and ran, perhaps he needs to be in better balance to be able to pick himself up more, lots of work on getting a rhythmic canter into the fences and trying not to pull and make him flatten.
Sally T is ace, she has 5* training at Truxford this Fri.