Welcome to the forums. If the brake shoes are good, you probably need to remove the brake cam, clean it and grease it. Also grease the pivot points for the brake shoes, but don't use much grease, you don't want to get any on the brake shoes or drum. I doubt if anything is wrong with the spring. I have seen this problem a lot with the Fourtrax 300. If you can't get it to release, you can buy a spring and hook it to the back of the brake lever and stretch it and hook the other end to the part where the trailer ball mounts, on the very back end. There are two holes back in the end of that piece and you can hook the spring in the hole on the right. That will make the brakes release. If you don't feel like taking it apart again to fix it, I would just add the spring, it works great.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.