Great question! If a campaign starts late due to a delay such as samples, talking points or something relevant to the brand, you will be rebooked on a new campaign and those impressions and earnings will carry over into a new booking. Simply submit for what was delivered in the date range for that campaign.
If the campaign started late because you were on vacation, had a personal emergency or something came up unexpectedly, then just simply deliver if (or what) you can and get paid accordingly once the campaign is over.