You must have waited for over 7 days before making your additional payment since the first.
Tier is only recalculated at the time of making a new payment, and your qualifications are only re-examined at that time.
You can add another $5 after total elapsed time is met for a tier (7+ days since the first payment for tier 2 or 3.)
Tier 1 will have a maximum balance of $100 also. That may prevent you from purchasing more credit until you spend down the existing balance with usage.
https://platform.openai.com/docs/guides/rate-limits#usage-tiers