I'm a little surprised that you're having this problem. Are you using the thin (1 mm) spacer needed at the back of the 10-speed cassette? A 10-speed cassette is narrower than an 8 or 9-speed cassette and needs that spacer when using an 8, 9, 10-speed compatible freehub.
Another possibility is to tighten the low limit screw on the derailleur to keep it from moving too far in. I would assume that you have already tried that.
I think a 10-speed derailleur has a narrower cage than your 7-speed, and of course that would be a possible fix.
Al