You should be able to tilt one side of the awning and there should be a set knob to tighten the arm down. Same as your manual awning, if you gently push (upward) on the arm and then tighten the knob, you can remove much of the sag. However, when it rains, you should lower one side so the rain will run off one corner.
If you're does not have any knobs to tighten the arms taught, then the awning should be self-tipping. Once it gets so much water, it should tip and the pooling water should run off. Mine does, but I don't like it, because once the water is run off and the weight is gone, the awning raises back up with a loud Ker-thump! It sounds worse than it really is. So I simply use the knob and tighten one arm in the down position. Works very well.
BUT! If retracting the awning, make sure those knobs are loose again and not tight.