Old Lazy Boy recliner won't stabilize? Where to look for problem?

I have an old Lazy Boy recliner, and a problem with it now. I can lock the foot rest into position, that works ok. But, the chair still rocks front and back when I have this foot rest lifted. Up til now, when you leaned back and pulled up the footrest, the chair stabilized, didn't rock.

As it is, the chair leans forward when the foot rest is up. Not so comfortable.

Any idea what component of the chair is responsible for this problem? Any idea where to start?

Thanks kindly.