Welcome to the Apple Support Communities. After reading your post, I believe that you may be looking in a specific account for notes. You can click the Notes icon in the upper left hand corner of the Notes app. You can also check out the help guide on your phone. To view the guide on iPhone, tap the iPhone User Guide bookmark in Safari or go to help.apple.com/iphone. I have also outlined some of those options to find those missing notes for you
Use iCloud to keep your notes up to date on your iOS devices and Mac computers:
• If you use an icloud.com, me.com, or mac.com email address for iCloud: Go to Settings > iCloud and turn on Notes.
• If you use a Gmail or other IMAP account for iCloud: Go to Settings > Mail, Contacts, Calendars and turn on Notes for the account.
Choose the default account for new notes: Go to Settings > Notes.
Create a note in a specific account: Tap Accounts and select the account, then tap to create the note. If you don’t see the Accounts button, tap the Notes button first.
See only notes in a specific account: Tap Accounts and choose the account. If you don’t see the Accounts button, tap Notes first.
Have a good one,