Wheel nut explanation?

I bought second hand single speed bike recently and I'm having a bit of trouble with the rear wheel.

When I attempt to remove the rear wheel from the dropouts I can get both nuts loose, but it gets to the point where both of nuts just spin and don't loosen any further and I'm not sure what the problem is. Could anyone tell me why the nuts is hollow as well please, I've never seen something like that before.

I've included a picture below (excuse the state of it, I'm still yet to give it some TLC).

Also, when I first got it the rear wheel seemed to be rubbing on the rear brake mount and preventing me from riding it, a service at the LBS seemed to remedy this but I'm worried if I get to the point where I can get the rear wheel removed that the problem will reoccur.

Thank you guys in advance. :)

It won't let me upload a picture unfortunately :(

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Sounds like the wheel is too wide for the dropouts. Not totally a problem just need to encourage the wheel to come lose of the frame. Pulling the stays apart gently is all it needs.

remove your nuts, take pictures of them and post them up here

(not grooming, honest!)

I've managed to sort it out now. The hubs were loose, a quick tighten and everything was right as rain.