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Has the rear ever been hit? It may throw off the alignment tube, so the handle or extension doesn't line up with the anchoring mechanism which lowers the tire.
There's a low-tech solution so you can access the tire anchoring mechanism. Gently remove the plastic guide tube between the inner bumper and the anchor drive. Then you can see the lowering mechanism and line up your handle or extension to it.
If the tube refuses to budge, yanno-screw it, just rip that sucker outta there.
I bet you're glad you found out about this little problem now, rather than on the freeway at night in the rain.
Please be sure to post again when you successfully remove the tire.