and in Sarajevo there are more officials and ball kids on the court than spectators in the stands
I have never been to a challenger but I have the feeling that people are rather working at this hour than going to a tennis tournament. Even at the bigger tournaments it is like this. I even think that in Umag the stands would be empty as well during the week if they didn't start playing in the afternoon like they do. The last time I went to Stuttgart, the stands were packed with school kids because of the school holidays. The "older" people were nowhere to be seen until later in the afternoon.
In Sarajevo I wouldn't blame the lack of good players though. There were enough local players which should attract the audience during the first rounds and now there are still some big (challenger level) names left.