Nerves/ nervous anticipation, possibly and because of it you aren't riding positively enough to the first fence. When i started eventing the beast, I had the same problem (albeit he couldn't knock a xc jump so he used to nap/ run backwards from it)
You need to ride your canter more positively - often a common fault. Try cantering immediately as you go into the ring. If the canter feels a bit nice and pretty, take it up a notch and that’s your sjing canter.
What is your warm up routine? What do you do when you go in the ring? Have you tried...
Jumping a max height vertical then straight in (literally- jump and try to keep moving into the ring and go as soon as bell goes)
Jumping a max height square oxer then as above
Before bell goes get a really punchy canter- do a few walk-canter-walk transitions or extend and back in the canter
Just like everyone above...I can hear my trainer when I was much younger shouting at me “ ride it like it’s the last fence!”...he may have sworn a bit too!