Some budgie females may sit too tightly but if she's feeding, there's really nothing you can probably do. There's a possibility that there may be an issue with the nest itself. She's a first time mom and might do ok with the other chicks if they hatch, but you might not want her to breed again (particularly if this continues in this brood). For now I'd try to not check on what's going on inside their box for a couple of days. If you hear babies calling, it's a good sign.
Breeding birds can be interesting but, we have to understand that when the parents successfully raise a brood, you're the one responsible for providing good homes for those chicks. Good luck with the remaining eggs.