Candidates from candidate pool do not show up on the audience breakdown in campaigns

We have created multiple candidate pools and there are thousands of candidates on each candidate pool. The candidate opt in field is marked as Yes for all of these candidates. However, when we create a campaign and select the candidate pool on the Audience section, the audience breakdown appears to be empty or in some cases there are only a handful of candidates.

Any assistance on why the candidates record don't match while creating a campaign is appreciated.