Og: image sometimes works, sometimes not

I need support for the "og: image" tag, it didn't work properly for a week. Sometimes Facebook will show a thumbnail, but other times it won't. I also tried clearing the cache using the debug tool, but that didn't fix the problem.

Here's an example of my code:

http://ogp.me/ns# fb: http://ogp.me/ns/fb# article: http://ogp.me/ns/article# ">

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