I too hated these caps at first. Why it's usually just the oil cap only is beyond me.
I took these pics of a gas cap just now, so I wouldn't get oil all over my fingers.
One day after getting pissed off about it. I thought...OK let's check this out, something is wrong here and it ain't crap in the threads or over filling.
Here it is guys...
The cap should look like this when you put it back in.
When it is giving you trouble, look at it and you will see it will look like this.
One counterclockwise turn holding the top and bottom will snap it back together and it will go right in.
Now my help and I know what to look for and never have a problem with them.
When you do get it to go on right, it's because without knowing it, it popped back down in place.
Hold the cap with both hands and turn it and you'll see what it's doing and what I'm talking about.