It will take a long time for the young birds to adapt to your farm if the lighting condition is different. That is why some farms could attract many birds in just couple of months and some may take years to do the same. This is only the Darkness and lighting parts there are many more things to be considered like RH, Temp, Flying Path, Sounds, etc. All these have to come in one, all must be at the best level any of the above is out of proportion where one is imperfect then chances of becoming a good farm will be reduced. So it will be good to look around the area where you want to invest and check around with the locals and gather some information. It will be a useful tool.
That is why many successful farmers in a particular area and later when investing their 2nd farm in a new place and later discovered that their 2nd farm failed even tho they follow100% their 1st farm in term of design, darkness, entrance hole,etc.
Well I guess it works the same for human too, It is just like you have been adapted to a certain conditions for many years and suddenly you were given something different (condition) or asked to change, so you might not like or you may take time to change. So this goes the same for Swiftlet.