Help needed : Simbar Bike Rack

I've got a Simbar bike rack which I've used about 6-7 times with no issues. It's not been used for about a year and has been stored in the shed all winter. I went to put it on this morning and it now doesn't fit! It no longer slides onto the bit of metal that sticks out from the spare tyre. I've tried WD40 and tapping it on with a rubber mallet, but it won't go on. Guy in bike shop reckoned it might have gone rusty inside. I can't see any visible rust tho. Anyone else had this issue and did you resolve it? I need my bike rack to be working by Monday morning for a planned trip. Please help!