Removing a Yaris tire cover

[SlothIV]

Tried the search feature, and I got a lot of information about removing the cover in the back over my spare tire, but none for what I need now.

I need to take the decorative cover off of my wheel to retrieve the air valve cap that fell behind it. The manual tells me to insert my pry bar and pull, but when I did, the cover stretched, but nothing seemed to loosen, and the cover went right back to where it was before. The obvious answer to "What should I do next?" is "Pull harder", but that's also the answer to "How can I break my tire cover". Is there something I'm overlooking, or am I being overly cautious?

[ET ECHO]

The wheel covers are on pretty snug. When you pry and let go they just snap back in place. Try prying in two places at the same time and see if it releases. Having a right angle pry bar will let you pull the cover off.