A HubSpot campaign groups the assets that serve one goal: emails, landing pages, blog posts, social posts, ads and more. When those assets are associated with the campaign, HubSpot can roll up their performance and connect the contacts who engaged with them to later deals. Attribution reporting, and how much of it you get, depends on your subscription.
Attribution models decide how credit is shared. First touch gives all credit to the first interaction, last touch to the final one, and multi-touch models split it across the journey. No model is the truth; each answers a different question. Pick one that matches the decision you are making, and keep it consistent so trends are comparable.
Credit goes missing for practical reasons. Assets are created but never associated with the campaign. Links in emails and ads lack tracking, so visits arrive as direct traffic. Deals are not associated with the contacts who engaged, so revenue cannot be traced back. The result is campaigns that look like they produced nothing.
Make campaign association part of creating every asset, use consistent tracking parameters on external links, and make sure deals are associated with their contacts. Review campaigns with zero attributed contacts or revenue every month; most of them are setup gaps, not failures.