Yep, definitely wrist work would be good. For maximum effect with least amount of effort, you might consider strengthening your fingers, wrist and forearm rotation. That would turn your arm and hand into a dangerous weapon on a badminton court
You may also feel that there is a problem with stroke production, even though the actual problem is in the travel phase (footwork) before the stroke: in this case your positioning and balance may not be optimal for smashing.
If you are behind the shuttle, well balanced, hitting the shuttle using a good whipping action and all this takes place between the net and the doubles long service line, it should be quite difficult for your opponent to return all your smashes.
Well, one more issue comes into my mind, though. This happens to me every now and then because players at my level can read my strokes quite well.
Namely, if I am smashing and trying to achive 110% of my max power, my opponent can return the shuttle and possibly regain attack. However, if I try to achieve only 90% power and use the remaining margin for clever placement of the shuttle or to build some kind of deception, then I will be more likely to hit winners...