Either not enough taxes were withheld or there could be a typo. Try these things:
* Double check that you actually typed in what is on your W2.
* Compare the W2 to your last paystub. Is the W2 number close?
* Check with your accounting department to see what you claimed. You may need to change your withholding.
* Read the state filing instructions posted below. There could be something else triggering your issue.
NJ resident instructions
DE non resident instructions
This link on filing non resident returns may help as well.