"I couldn't finish my homework because I didn't understand it!" (or some similar variation)
I personally do not assign homework just for the sake of it; more often, it is for students to practise a new skill or to move more quickly through a unit that should not require as much direct instruction.
When 'stuck' try:
- asking a friend / classmate / peer
- asking an older sibling or parent
- reviewing notes in workbooks and/or text books
- looking up answers on-line
- e.g., from this past weekend's homework, some students did not know what a 'reflex angle' was (even though there was a mini lesson on Thursday) --> 'Google it': "what is a reflex angle?"