Main reason being they cannot find or figure out the path to go downstairs.
If your LALs are well placed you will find the reverse things happened inside your BH.
Those birds will populate your lowest floor more than the top floors.
I was looking at this strange happening at Sedili BH where one of the monkey house dedicated to pull those birds to the middle and ground floor.
Nearly all that entered stayed on the ground floor.
The owner wanted to know why and he prefer his second floor to be populated too.
There could be two possible reasons for this case.
1) The sound volume, both internal and external, on the middle floor were both too low. I was right and managed to adjust the both sound volumes.
2) The path going downstairs was properly place and fitted with proper external sound tweeters to pull those swiftlet down.
This was how a totally new LAL hole was opened and decorated with external tweeters.
My workers installed about 12 pieces and orientated as such that any birds that flew down will be attracted to go one floor further.
A nicely arrange tweeter might be the true reason.
I am a kind of looking around for something special and this might be the one in this BH.
I know now that if I want them to come down you must make sure that the LAL is properly placed, proper size and properly installed with external sound tweeters.
If your BH is currently facing problem where most of your birds refuse to fly down try this idea.