Hi, I'm Sandy and I look forward to help you today.
I'm sorry about the issue.
Have you logged into your webmail acount like gmail account or other account and try to find that missing notes there?
If not, please try to login into your wembail accounts, and mainly gmail account which you have used while setting up this device and check if you see your missing notes there.
Please keep me posted and reply back to me. Thank you.