This may be happening because you used Invite via Email for a person with an existing Behance account. Inviting a Team member via email is only for people without existing Behance accounts. If you send Invite via Email to someone with an existing Behance account, they won't be able to accept the invitation on their profile.
If you'd like to invite a Team member who has an existing Behance account, type in their exact name or username to find them via Invite a New Team Member.