The Association is aware that many alumni paid partial amounts as part of their School fees without this resulting in full Association membership. Membership requires both a completed form and receipt of the full fee — the Association has no reliable way to match partial payments collected by the School to individual alumni unless the School shares that detail.
If you believe you paid but aren't listed, the Association's advice is to register and pay the life membership fee now via the Become a Member page. If the School later shares confirmation of a prior part-payment, it will be refunded to you.