It is possible that the other eggs are infertile (especially if this is their first clutch). Normal incubation for budgies is usually 18 days. Since there's no way to know which egg was laid on which day (unless you marked them), do not remove them until at least 20 days after the last egg was laid. When you do remove them, hold them close to a bright light (this is called 'candling') so that you can see if there's an embryo inside. If there is, it may just be slow developing.
If this is a young pair and this is their first clutch, you will probably find that fertility rates will increase substantially with subsequent clutches.
Good luck, and enjoy the babies!