IGNOU is a massive university that has millions of students from all over the world. Working on such a large scale, it is subject to some delays sometimes. If you have waited for more than 2 weeks, visit your Regional Centre to know the exact cause of delay.
You might have failed to secure minimum marks and will have to redo the assignments. Following are some very important points you need to ensure before submitting your assignments:
Get your assignments photocopied prior to submission
Take the signature of the person accepting the assignments
Keep the acknowledgement slip safe for future reference
By doing this you will have a proof of assignment submissions in case a problem arises.