Why didn't a recently unsuspended customer get billed?

The most common reason is an account billing configuration for what to do "when customers are un-suspended."

The account level setting can be changed in the Admin UI under Settings, Financial, Billing Preferences. 

"Charge for all missed periods" will generate an invoice every hour for each missed period. 

"Charge for current period (Prorate if applicable)" will charge for the current period from the day they are un-suspended until the end of the current period. 

"Charge for current period (Do not prorate)" will charge for the current period from the start day of the current period for the full period amount. 

"Do not charge for any missed periods" will skip charges until the next billing period arrives.