My class APC load cap went down and said trader buff activated. What should I do to make load cap back..

Sounds like you changed class to Trader. Unless you want to change back then to increase your load cap you need to have more troops in the APC. (upgrades and heroes skills). Or complete research for each of the APC research trees or under each of the troops type research trees to find the items that improve load capacity. Use all vehicles in the APC as well to increase its load.