Duncan's are typically found in lower light areas, but are adaptable to higher light. Since your's is growing so well and others that require more light are growing quite a bit slower, it's quite possible that your light level is the major limiting growth factor (assuming flow is correct for the various species). Many BN morphs (but not all) like a good amount of light as do Montipora. Trumpets and Pipe Organ like light too, but a bit less than your SPS.
The old say 'Be careful what you wish for' applies here

Once stony corals really take off keeping up with their calcification demands can be a challenge. I can usually get away with using a saturated solution of Kalkwasser. But when things are really growing well, the system gets close to the limit of regular Kalkwasser and once or twice I've had to use a bit of vinegar to make it just a bit more potent without resorting to an additional product such as a 2-Part.
True mixed reefs are tough and in many ways the smaller the tougher (less space for light/flow variations). Even when everything is going well, there seems to always be one or two corals that are just 'grumpy' no matter how thoughtfully they are placed. I've found the best thing to do when starting is to first follow a general 80/20 rule, so get 80% of the corals 'happy' and then slowly work on the 20% of the grumpy ones by slowly moving them to potentially better locations and your time (weeks) to observe any changes (assuming you don't see an obvious problem like continuous polyp closure, loosing flesh, etc.. The worst thing to do is to start fiddling with the system's light and flow when most of the corals are happy just in order to try and improve the condition of a few grumpy corals.