There is a common reason for this:
Most likely you have selected "Opt-In" required on your online form, which requires the contact to confirm their email address with you before it enters them into the system.
If they have not confirmed the email, the contact 1) Will not be added to your system 2) No automations will take place 3) They will not be tracked on your website
If you'd like remove that Opt-In being not required, you can edit that within your settings for the specific form.