My "Yellow Notes" are disappearing! Help!!

You know the Notes thing on your iphone that you can make lists on? I've started using this instead of paper for grocery lists, to-do lists, etc. It's very helpful and I don't lose them.
However, I have had some of these lists disappear without deleting them. I thought the first time, it was something I did. I just had it happen again for the 3rd time! This is unreal! I had (twice) my to-do list (housework, bills to pay, meetings! etc.) disappear. Very frustrating! My memory stinks and I can't remember the stuff on it to re-do it!
Any ideas why this is happening?
Please help me!

Are these notes synced with iCloud to another device?

chas -
No. They merely replace a piece of paper in my pocket. I do not sync them, try to access them elsewhere or anything like that. When I think of a grocery item I need, I add it to my store list. When I think of something I need to do when I get home (pay bills, fix screen door), I write it down. When I complete a task, I delete the task, not the entire note. If I do delete a note, it should only delete the note that I am trying to delete, no?
Anyway, I've had 3 occasions of multiple notes disappearing. This last one yesterday DID happen after I deleted a note, but for some reason, 3 others were gone the next time I opened Notes. I don't recall if this happened before my previous ones disappeared.
Is there a glitch in the app/software that causes this to happen? There should only be one note open at a time, so multiple notes shouldn't disappear.
I'm very frustrated because I rely on these as my memory - I don't use the Reminders.

So you are saying that if you write a note down on let's say your iPhone, it is NOT PRESENT on your Mac? I'm just trying to get a clear picture of the situation here.

Also, why not use Reminders, as that is better-suited for the task anyway?