There are some particular reasons that are responsible for the disapproval of your application.The reasons are mentioned below-
1) If there is a problem with your contact details or name: If this is the reason of your disapproval then don't worry, sign in under www.google.com/adsense with the email address you first applied with. On the next page, you will see a form where you can correct your address data and resubmit the application.Remeber thing google adsense will allow only one account per particular user. So,make sure that the name or email address that you have given is not used yet.
2) Problems in your website: This is a great thing you have to know before you apply for google adsense account. Adsense have some certain policies and they follow them strictly. If you have any content in your website that is against the policies of adsense, they will reject your application. So, make sure that you have fulfilled all the policies or guidelines that adsense included. You can find out them in here: Policies
3)They will review everything of your website: Everybody knows that first impression is the final impression. Make sure that your website design is pretty good that will help you a lot.
Don't use any copy paste content (Plagiarism) on your website because it is against one of the policies of google adsense. So,it is also the reason for not getting approval.
4)Remove all the mistakes from your website then re apply
Peace be upon you