In the Quotes screen, the Status field helps to track the current standing of the Quote. When the Status field is changed to "Rejected" the Reason Rejected field appears where you can select the 'reason' from a drop-down list.
In order to populate this list with more options, you'll need to go to Sales → Enter Reason Quote Rejected List.
To add additional reasons, click the Add New button and enter the reason. Then click OK. You can edit Reasons at any time by returning to this screen and double-clicking on the Reason you wish to edit. Note: If the Quote screen is open while you are adding reasons to the list, you will need to close and reopen the Quote screen in order for the new information to become available.
To get an idea of why your proposals may be getting declined, you'll head to Sales → Reports → Rejected Quote Analysis.