This usually happens when the tree leans towards the platform. Today they are stands out there that corrects this problem due to the ability of being able to adjust the platform itself.
It would be better to find yourself a tree that is more straight, with at least 8" in diameter and this would/should solve your problem. If it's the only tree around, nail a 10" piece (or two) of 2 X 6 bewteen the area where the 'platform' makes contact with the tree, this will adjust the platform up, but like I've said, because the tree is already leaning towards the platform ( of course, I'm assuming you've snoged the seat as close and secure to the tree at the beginning) your position may not be quite comfortable, because the angle of the tree tends to push your back away from the tree making for some discomfort.
Be careful--and remember to USE your safety harness when engaged in this exercise!!!