How can I fix my Suffolk Punch 35s roller?

Hi.

On my Suffolk Punch 35s the rear steel roller engages and works how it should when the engine is running.

However when I want to move the mower around the garden and out of my shed the roller doesn't move. I cant really turn it by hand due to friction pushing against it and stopping it. It used to move freely but all of a sudden it doesn't.

Any help would be greatly appreciated.
Many thanks