If the required movement of the lever is as large as you say, then it could be down to excessive wear at the pivot. I did change my lever a while back for that reason, but mine is a fairly high mileage example.
Alternatively, if the lever only needs a tiny movement from its resting position to change down and a huge one to change up it could be failure of one of the return springs at the other end of the shaft allowing the lever to sag. If so, you'll need to investigate under the clutch cover. If you're lucky you may be able to get the cover off without damage to the gasket, in which case it can be re-used, but it's best to have a new one available before you start.