Could be stalling the plane. Slowsticks fly slow, but improper cg, tail heavy especially, or too much weight can make the plane unstable or unable to pick up enough speed. If it is too much weight and tail heavy, then the plane will be nearly impossible to fly. It is important to note that a bigger pack, depending where it is installed could also cause tail heaviness.
Dihedral is important, especially on a plane with no ailerons, and even more so on a slower plane/a plane with a low rudder throw. From my understanding, rudder will decrease the lift of the main wing on the side the rudder turns to, as well as forcing the rear end to deflect to the rudder, and as such, rudder will cause banking, and the more rudder, the more it banks, like my UMX Pitts, which needs a lot of aileron and elevator to counteract the rudder, so that it can turn level. Dihedral will counteract rudder too, as when it banks, the side it banks to will gain more lift, while the side it banks away from will lose lift. If you have low dihedral, lift will not change much on either side. Likewise, say if the plane banks on its own from wind or improper trim, rudder to the opposite of the bank will lower airspeed on the higher wing, and will help leveling. If you have low rudder throw it can't help the situation. In essence, low dihedral and low rudder throw will make the plane unresponsive to unwanted banking, and hard to level in banks made by the pilot. As your rudder is the only steering in the plane, you best have a good amount of throw to it, of course with expo, so it is smoother. Ailerons clearly are more effective at banking, so if they aren't around, you have to let rudder do it all for you.