M21 drive

I got an M21emrb SN is J288c0080
I greased the drive wheels the other week and now the drive is really weak.
It will pull the mower on the flat but will not pull it up a small incline.
When this started to happen sometimes the drive would quite and when I
would try to pull it backwards the wheels would lock up.
Any Idea on how to fix this problem. It was fine than this.
I only use this mower for about 15mins a week to mow a hill.
I bought a new snapper but it does not have posi traction so I use this for the hill.
Also, anyone know the year of this thing.

Thanks Brian