Can't understand it when you don't notice (4,2,3)

'you don't notice' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'can't understand it' is the wordplay.
I cannot quite see how this works, but
'understand' could be 'see' (synonyms) and 'see' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
This may be the basis of clue (or it may be nonsense).

'when' is the link.

Can you help me to learn more?