Ads not shown sometimes

I've seen that some of my keywords do not trigger ads at
all times.
They seem to go off and on. After sometime, say 2 hours or
so, the ads
get triggered automatically and it starts to work again.
I've made sure
these keywords are active and they meet Google's standards.

Has anyone else come across this before? What could be the
possible
reason for this?

KoC


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