NH 648 Floor drive roller

My NH 648 string wrap machine has a problem. 

 

See Pic below, the Floor roll drive chain ( 5 ), is held tight by a spring loaded idler ( 6 ), the chain is new last year, the spring loaded idler won't stay in the chain. I found this when a hyd line broke a few days ago, I used a pry bar to lift the idler and put it back in the chain, assumed all was fixed, but it isn't, the idler will climb out of the chain as soon as I engage the pto, the spring is adjusted right and it holds the chain tight but won't stay in the chain.. the leaver it self isn't bent or doesn't look bent, I haven't removed it yet to be sure.

 

What would cause the idler to not ride in the chain??

 

Thanks, Chris

 

NH 648 Floor drive roller