I opened up a new AdWords account a few weeks back and had a similar experience. I got a few exposures at low rate for a short time. After that no more exposures.
I wrote adwords support and a day or two later someone reviewed the account and enabled it for full rate exposure of ads.
No similar problems since then.
I've run into this problem in the past - it will almost certainly have been caused because you've attempted to use some keywords that have been flagged by Google and that they need to check manually. For example, some copyrighted terms cannot be used as keywords.
Suspect that may be the problem.