Apple pencil markups disappear in Notes

Hello,

I have been using my Apple Pencil to markup PDFs that I have saved in Notes. On two occasions I have gone back to the note and all the annotations have disappeared. Does anyone have a remedy to this or more importantly a way to recover the missing markups? I took notes for a very important exam that you could only get by attending a certain course and now I have no way to access them. Thank you in advanced.