Most applications for IDR plans are now submitted online. Once logged they are either given the designation of a completed or partial application. If complete the application begins processing and if partial the borrower is notified and the application cannot be be processed until everything is received to complete.
The problem is that online applications are done through the Federal Student Aid website. As a result when your servicer receives word, they only receive whether it is complete or not, not what is missing. To determine what supporting documentation you must provide, you must go back to studentloans.gov where the original application was submitted, and log into your account with them. From inside their website you will be able to view what information is required.
Unfortunately this is the only way to do this. Because applying is not done through your servicer, they will be unable to tell you what information is missing.