Hi Kylie, thank you for your answer. Will give it a try with the excersice you proposed. The horse is 7 yrs old and is really good. It's most of the time 1 fence down due to this problem.
Follow up --- you could make the one stride exercise I mentioned with a caveletti/raised pole in the middle for an extra bit of difficulty, it'll make your horse really wake up and realize - hey I need to get myself together.
Do lots of bounce grids with oxers (not at full height that you compete at - maybe 3'6"/1.05m).
It is also good to have a one stride line set up, and you make it as follows: jump, bounce to a pole, bounce to the second jump. So really a one stride that you have a pole in the middle of.
It is alway good to go back to basics and practice many shortenings and lengthening of your stride.
In addition to this, you may want to do some hill training.
Out of curiosity, how old is your horse?
I've never jumped that high, but maybe doing exercises with ground poles that makes your horse lengthen the stride could help you, and then doing the same with jumps smallers.