If you have trouble getting a decent left-handed stroke using traditional grip, there are a few things you can do.
Take both sticks in one hand, palm up, and rotate your arm a full 180 degrees so your palm is facing down. Do this over and over again. The movement shouldn’t be too fast, this is just to stretch the muscles. Even without sticks this’ll help your wrist rotation.
You could also do a stretch like this…
…where you hold your hand out, palm up again, and grab your thumb with your right hand. Gently pull your left hand downwards by the thumb. You should feel a slight stretch in your wrist and upper arm.
Another easy thing you can do to improve your left hand in traditional is left-handed buzzes on a drum or pad. Pushing that buzz into the drum will strengthen your strokes if done correctly, so make sure that your thumb is driving the stroke and that you’re pushing the sound through the drum.