The CORRECT procedure is to inform that NCO's failure to follow through on a task. Let that NCO's supervisor handle this.
It is NOT a junior Soldier's place, regardless of position, to dicate or correct an NCO for "not completing an assignment on time..."
Additionally, one must also take into account what other responsibilities this NCO had. Perhaps there was a reason for this "failure"
" Excuse me, SGT/SSG/SFC (?), I noticed you didn't complete the task I gave to you a <insert time frame here> ago, is there a reason for this? If possible, would you be able to complete this by <insert time frame here>
That's how you should handle it.