They have been known to get very stiff over time, to the point were the mechanism can break or, if you’re lucky , maybe jump a tooth on the internal plastic cogs. This would cause the front and rear to be out of synch with each other.
Was the lever on yours previously stiff to operate? If so it might have jumped without you realising.
You might be able to fix it by stripping it down ( remove the 4 screws in the bottom of the box, 2 on each side, which then allows the lid and lifty uppy bit to be removed from the base) and cleaning it all out. The issue is partly due to dirt and grit build up between the sliding parts.
You might be able to spot if the cog teeth have jumped and set them to the correct place before reassembly.