There could be a couple of reasons why you might be experiencing a delay:
1. First, the status might be pending for security reasons. This may be a sign that someone is trying to use your identity to file a tax return. This can take up to 72 hours, after which your return will then be forwarded to the IRS, or if applicable, rejected. If rejected, you can then correct the rejection issue and resubmit. Please see this FAQ for reference.
2. Secondly, there could be a delay if your return has various refundable credits, such as the Earned Income Credit, Investment Credit, Energy Credits, or Business Credits. You may need to wait a few more days and check your filing status again. Please see this FAQ for additional information.
Note: Both instances may be applicable. That is, you could have a hold for security reasons first. And additionally for verification of the refundable credits in your return. In this case, delays might extend beyond 72 hours.