A :comment points at the words in its brackets. With nothing in them it has nothing to point at, so it renders nothing. Put the words it's about in its brackets, or answer a thread with reply.
Incorrect
The link expires after 15 minutes
The link expires after 15 minutes :comment{note="Is 15 minutes enough?"}.Correct
The link expires after 15 minutesIs 15 minutes enough?.
The link expires after :comment[15 minutes]{note="Is 15 minutes enough?"}.