Don't quite know what you mean, esp with the deer comment, as they are good jumpers?
Is she whacking her knees on the jump? If so, don't jump her until you get to the bottom of it, as she can become injured quite badly, not to mention the risk of a fall.
Perhaps you're talking high jumps & she's not fit enough or well trained enough for this level yet?
Does she do this when you drive her over jumps, or just with you on board? If it's just with you, it could be a saddle fitting issue - perhaps it's over her shoulder blades or otherwise pinching or locking up her shoulders.
Or perhaps you're asking for too much collection coming up to the jump & she either is distracted by this, or the position she has her head means she can't see & prepare for the jump adequately.
What's happening is your horse is too on the forehand coming up to the jump, you have to lift him up and put him on his hindquarters, help him bottle up his energy, ready to explode over the jump.
Ground poles will be good to help your horse judge a earlier take off point so he isnt leaving things to the last minute, pull the ground pole out away from the jump to the poin the horse SHOULD be taking off from
A good exercise is to set 2 oxers, and allow one stride in between. Make the first a ramp oxer, and the second a reverse ramp oxer.
(A ramp oxer is when the first bar is a few inches lower than the second, creating a "ramp". A reverse ramp, you guessed it! Higher first bar).
The ramp oxer will encourage the horse to bring his knees up higher, and keep his lower leg tighter. As a result, he will land deeper after the first fence, causing him to need to sit back and rebalance for the second. The higher rail in the reverse oxer will cause the horse to sit back and judge the fence "measure" it, if you will) which sets him up for a tighter front end over the jump.
But, as jeddah said, just getting him off his forehand first will help the problem. :)
I always give my horse a gentle squeeze every time I go over the jump. It helps him pick up his feet and jump a little higher. But it seems like your horse is front heavy. She heavier in the front part of her body then the back near the hindquarters. This makes it a little harder for your mare to jump.