I have had this problem now twice - both times I thought I maxed the cards, but it turned out to be 'security getting worried' - this is a big problem with myspace ads, seemingly once they decline a card on your acc, even if you get that card to go through again, they stop the impressions. They stopped impressions on me in the middle of what was still a $9k day!
The only way I found of getting over it was to create another account and start again. It sucks balls, as I had to wait for approval etc et again, costing me a lot of time and money on what was a successful campaign. I've emailed them several times asking them to turn the acc's back on, they emailed back once saying it was all fine - still no impressions.
Moral of the story? Always have a backup account with ads ready to run! The issue needs resolving though at their end, its frustrating as hell.