Remember Adwords placements are dynamic with keywords being bid on all the time, so keyword cpc will vary all the time. It may require 30p to display one day, and maybe 20p another. If you're budget for that keyword is say 25p, then when it rises to 30p, it will stop showing.
I have keywords that regularly display ads, then later the same day stop displaying for this reason, or another keyword in another campaign is more relevant (which isn't great as I'm competing against myself
)
Does that help?
