Check the bushing for wear as Albie said, and also check if you have an OEM countersprocket on there as the aftermarket ones are sometimes too thin.
Otherwise it's probably the O-ring as Chris said, they flatten out over time, especially if you unbolt the countersprocket and put it back on.
I like to use the O-Ring from the two strokes, it has very similar outside dimensions but is a bit thicker and seals better. Don't have the part number handy but you should be able to find it by searching here and on KTMtalk.
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