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It can't be because my CPC is to low because the max cpc is very high, and it can't be because my daily budget is to low because I've never got close to it, and these keywords have been in the ad group for more than two weeks, some for a couple of months.
Am I the only one with this problem?
If I had to guess, I'd say it had to do with some keywords being duplicates of others. For example, in the eyes of the system all of these keywords below are the same. If you had all three, it's quite possible that only one of them would ever get impressions:
helium filled cats
cats helium filled
filled cats helium
BTW, are any of the zero impression keywords marked 'disapproved'? Are any of them in regards to potentially sensitive subjects?