This page of our Assassin's Creed Valhalla River Raids guide explains how to increase your boat capacity and what to do if you can't collect anything anymore.
Return to the camp
The most important information is that while you are performing raids, you can return to the camp at any time. Select the option from the additional actions menu. You launch them by holding down a specific key.
After selecting the return option, your team will simply return to the boat and head to the camp. You will take all the raw materials you have collected with you.
You can go back at any time to collect the rest of the raw materials, treasures, keys and other types of resources.
You can increase cargo with Foreign Supplies. You can obtain them exclusively during river raids, you can't get them by travelling across the standard game map.
If you get enough supplies, go to Vagn and choose the option to customize ship.
You will see Cargo on the left. You can expand it twice. The first expansion costs 450 Foreign Supplies.