Go to friend screen and back to synch with server. It should make the missing ones appear. Also if you have many characters not on task it tends to freeze them in place and may cause issue with more spawning.
ETA: They are attracted to the stadium. I clear all buildings and look for balls, by the time I'm done, they've collected at the stadium. The freeze is temporary, they will start to move after some time passes.
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ETA: They are attracted to the stadium. I clear all buildings and look for balls, by the time I'm done, they've collected at the stadium. The freeze is temporary, they will start to move after some time passes.