If the plastic fitting has an area that is larger near the tip (where you slide the hose on) this is refered to as a barb. They usually have one side that is sharp, a bit like a tooth, so that the hose catches on it when you're trying to pull it off. If this is the case, then the clamp needs to be up further so the clamp itself restricts the tubing from being pulled over the barb, or expanded area, of the fitting. If the fitting is just smooth then you are relying on friction only, which is increased by the force the clamp exerts.