Hi there @mdh,

Hope to help add some detail into what might be going on.

If you have created the custom property AND see that it is being logged under a user’s profile, then that leads to one possible error. However, my guess is that you are not seeing the custom property within a profile therefore we cannot segment from it. Adding a custom property doesn't automatically add this profile to the segment in question. In fact, due to the nature of how segments work, you simply cannot add profiles as you wish. The segment exists based on a set of pre-defined definitions or criteria. In this case, it's anybody who has come from your “source”. Unfortunately if the event source has not shown in this customer profile history they will not appear in the segment.  You can run a historical sync by going to your integration tab and click " re-import historic data" button to run any updates for your account. If the customer source value still does not show the only other alternative is to manually add the email to the segment. But we prefer and recommend for customers to be added to segments organically versus manually adding them to the segment. 

Once you confirm source is being loaded successfully as the custom property, then we can see what might be going on.

All the best,

Alex