When you see something is causing those swiftlet not entering your nesting room, you need to take some action !!!
This was the exact situation which my Kuantan BH was facing.
It seems that the moment they entered the monkey holes they cannot find the window to enter the nesting rooms.
The moment they entered they will be drawn towards a set of tweeters located just below those entrance holes.
Their main problem was that they could not find the entrance window at the middle of the middle wall since it was not precisely located.
Owing to this short sightedness I told myself that the only was to get more into the nesting room is to open a new hole that is precisely located based on their flying pattern (using CCTV camera video clip).
I choose a hole about 2.5 feet by 2.5 feet just below the beam.
My next important mission was to install at least 4 external sound tweeters to draw them in.
Prior to this only 20 -30% entered into the nesting room areas.
After the new opening the number swelled to 90%.
Have a look at these video clips and hope it will teach you some important lessons.
"Make sure every entrance door(s) or window(s) of your BH are precisely located. Get 100% of those birds that came into the monkey house will eventually reached your nesting room.."
Do not take for granted just because you thought so or your Sifu told you so.
It is the birds that can tell you better than any other consultants where the entrance doors/windows should be located.
Observe their flying pattern, using your CCTV camera recorded clips, and decide where to open and which one to closed too.