i would have checked Canada using a proxy server, but now that eveything is ok you can probably just put it down to a random glitch. On a few rare occasions I have also encountered the problem which you have described.
Have you set up alternative ads? If you let the alternative ad point to a white pixel, you see a white space where a PSA would be. If you have not set up an alternative ad, I would file it under "Random Glitch".
I've had problems like this, but only in one specific example -- that would be working in FireFox on a blog site that featured three ad units. Typically, the first would load up and the other two would either adopt the same properties of the first -- or simply not appear at all.
The best thing to do is take a step back and verify the behavior in another browser, through a proxy, etc. I've never seen an issue linger much once that's been tried.