If you are observant enough you will see some signs that those new birds are building their nests.
I was in Gambang Pahang yesterday and wasted no time in inspecting a BH that have a very peculiar problem.
"Pak Harry, every evening there are at least 150 to 250 birds entering my BH but up to today there are only about 20 nests. What can be the problems?"
From the number of birds entering and the number of nests currently inside, it seems to be very unusual.
I told him to be patience and perhaps gives more time for them to start their nest building.
During yesterday's trip I took a long look at any signs that those 150-250 birds left.
What I can see was that they are not leaving many markings on the floor.
The number of bird shit spots on the floor was too little and not commensurate with the number of birds that was seen to enter the BH.
If there are 150-250 birds I would expect at least 75-125 bird shit spots on the floors.
What I can see was about 50 or so.
There were a number of new nests under construction plus a number of new markings on the nesting planks.
So what can be the reason why they are not shitting?
I think they are not staying once the external sound stops.
The internal sound was not well like by those birds.
Or maybe your internals sound is dead during night time?
My recommendations for him were as follows:
1) Check the operation of those internal sounds. If you have two amplifiers try to test one by one.
2) If your amplifiers are working check which areas they are working and which areas they are not.
3) If possible try to change the internal sound to something that are proven to attract them to stay for example : BabyKing or SuperBabyKing.
4) Make sure the sound volume of those areas that you wanted those birds to stay be a bit louder.
Have a look at some signs that those birds are making their new nests: