Some of the reasons why your referee's status changes to Ineligible for reward are:
They did not top up at least $100 within 7 days of signing up
They topped up at least $100 but withdrew within 7 days of signing up
They topped up at least $100 but did not maintain it during the 30-day lockup period
Please consider topping up a little more than $100 if assets are non-stablecoins as they may go down in value due to volatility. Once the balance goes below $100, they are automatically ineligible to receive the referral reward