I didn't do anything to the front roller assembly and its still leaving lines so might try replacingspring in front in the assembly to help firm it all up.
My machine started leaving lines when I mowed, also had the problem of the rear roller spinning on a slight hill I have.
Being spring time I have just spent 2 days of my holiday scarifying my lawn back down to almost soil level, no more lines and no more roller spin.
I would consider dethatching your lawn
