Does this happen every memo that you type? Or just that once?
Do you have a signature?
There is an issue that, if you have a signature that includes blank lines, and start typing your memo on line '2' (within the signature) - spell check won't check what you type.
The fix for that is to add 'SpellCheckSignature=1' to notes.ini. Or ensure that you always start typing on line 1 - before the signature starts.
If you don't have a signature - ignore me!