There are a few reasons a certain client might not be in your auto-campaign:

Auto-campaigns are forward-looking by nature. If a client has met an entry rule prior to the campaign being set live, this client would not enroll. More on this here!

The client didn't meet the exact entry rule. Take another look at your entry rule to make sure it is set up to capture the clients you intended.

The auto-campaign was not live when the client met the entry rule. If an auto-campaign is inactive, whether in draft form due to never having been started in the first place or due to being paused, no contacts will enter during this time. If a client meets the entry rule while the campaign is paused, they will be skipped and won't enroll, even after the campaign is set live.