Think of your ipad as a filing cabinet with a limited amount of storage. If you fill up your filing cabinet, then nothing more can be stashed there until you toss some stuff.

If the cabinet [your iPad] is full, then Notability will NOT save what you’ve been working on.

How to tell how much storage you have used/have available:

Settings –> General –> About

How to tell what app is using the most storage:

Settings –> General –> Usage –> Manage Storage

Suggestions for clearing out some space:

• First thing to try: Close out of all your apps [doubleclick home, swipe up all the apps], then reboot by holding down the power button until the “swipe to turn off” slider appears.
• Second thing to try: Settings –> General –> Software Update. Click on the triangle at the bottom – it MAY let you delete the downloaded update file.
• Photos & Camera: Delete any pix/videos you don’t need. If you have some you’d like to keep, upload them to Drive and then delete from the iPad
• To upload pix/vids to Drive: fire up the Drive app. Create a folder where you’d like to stash the files. Click the PLUS icon. Select “Upload Photos or Videos”, then camera roll. Click to select the files you’d like to upload, THEN CLICK THE BLUE CHECKMARK at the top right.
• iBooks: delete anything that you don’t need anymore
• Notability: delete anything not needed. If you don’t need it NOW but want to keep it, upload the file to Drive by clicking on the Export icon at the top of the screen.

  http://tech2.mountdesales.net/blog/2013/ipad-how-why-backups/ has detailed instructions.

• Mail: Sometime Mail is taking up tons of room, especially if you never delete anything. Mail is actually downloading a COPY of your emails from the server – to clean things up without actually deleting the mails, delete the account from your iPad [Settings –> Mail]. When you set it up again, select “Google” for the account type.