There seems to be a issue with Simple Ads and Jetpack. I can get a situation where ads aren't displayed because by the time it gets round to displaying the post the advert count has already exceeded the max value set in the configuration.
Putting var_dump('adding 1'); into the increase function shows that the count gets incremented several times before we get to the post content. This stops when jetpack is disabled. When jetpack is enabled it only seems to happen when more than 1 sub module is enabled