Hi,

I took both wheels off to replace the tires. Job done. Re-installed front wheel with no issues.
Now the rear wheel - this one is driving me crazy, because I have no idea what I'm doing wrong.

I tried doing at least 20 times, and every time, the part of the hub that is secured to the frame - goes all the way in on just one side, but not the other. It's round but has two flat areas, so I align it perfectly on one side, but it's not perfect on the other side. How is this possible?

Here's a photo in which you can see the side in which the hub's thingy is not completely in the part of the frame that holds it (apologies for not knowing the terms).