One thing stuck out: your rear foot's heel is off of the ground. Which means if you're going to get any power from your back leg, you have to push off of your toes. This is not good mechanics. For example, try to serve while putting no weight on your back leg [ie lift your foot completely off the ground]. This is partially what you're doing.
High-level servers in a platform stance raise both heels off of the ground but not nearly to the extent you are; check out Federer, Dmitrov, Pospisil, etc.
What @Dimcorner recommended, putting more weight on your back foot, will be extremely difficult the way you're currently standing.