After a campaign has ended, it’s not uncommon for some pledges to drop. In these cases, we do reach out to backers with reminders to correct their pledges. If a backer doesn’t resolve their payment issues within seven days from your project's deadline, their pledge is dropped. This means that they will not be able to update their payment and are no longer considered a backer of the project on Kickstarter. There is no way to extend this timeframe to fix a pledge, nor is there a way for a backer to support the project through Kickstarter after the seven days. You can read more about this here.
Please note: Kickstarter will only collect fees on funds that are successfully collected.