MTD Self-Propelled Mower - Can't Change Speed

Hi all,
I've got a Cub Cadet (MTD) self-propelled SRC-621 walk behind mower. It has 6 speeds. However, I can't change gears. The lever won't move. Sometimes, the rear wheels lock when pulling the mower i.e. out of the garage. Has anyone experienced a similar issue and can offer a solution or advise? It does get annoying at times.
I bought the mower over 5 years ago. It's always been a good perfomer. I've never had any issues with it; except, for this issue and the plastic wing nut holding the wheel axle came off leading to a bent fork.
Thanks in advance, Orlanzo
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