Illinois: why is my paycheck lower than expected?

Top reasons a Illinois paycheck looks smaller

1. New W-4 (federal or state).
2. Year-to-date wages crossed a bracket (less common in flat-rate states like Illinois).
3. Bonus withheld at supplemental rate.
4. Benefits enrollment change.
5. 401(k) percent change.
6. New garnishment or child-support order.
7. Work or home location change triggering a local tax.