The continuous shows an action in progress at a stated moment, not merely an action that lasted.

The continuous frames an action around a point in time. Without a point — stated or implied — there is nothing for it to frame, and the simple form is the right one.

At nine last night she was drafting the reply.

a moment is named, and the action surrounds it

At nine last night she drafted the reply.

grammatical, but says it started and ended at nine

Where it bites

Where the sentence names a time. 'When the bell rang, I was leaving' and 'when the bell rang, I left' describe different orders of events.