Hello @MarcSimps,

Welcome to the Klaviyo Community!

I think it’s important to understand that not every contact in your account is going to be considered a subscriber in your list. This surplus of contacts (profiles) that end up on your list are synced through general engagement from any point of providing you their email address. We explain more on this in our Understanding active email profiles in Klaviyo help center article.

Similarly, not everyone who fills out your form is going to end up on your list as subscribers. This behavior is typical if you have double opt-in enabled on your list - which is by default. Users who fill out the form, but do not confirm their subscription would still be synced to your Klaviyo account as an active profile. Only until they confirm their double opt-in would they be considered subscribers and added to your Klaviyo list. 

With that in mind, I think using the segment definition “what someone has done, placed order zero times over all times AND properties about someone, $consent_form_id equals X” should do the job at grouping users who filled out a specific (Klaviyo) sign-up form but has not made a purchase.

I’ve also included some additional Community posts below that I think it may be helpful in taking a look at:

I hope this helps!

David