Don't read into things.
Sometimes you won't get a call back for a couple of days because the person you're trying to contact is busy. Sometimes you won't get a call back because they're out sick or taking a day off. Sometimes they find out that the hiring manager already filled the position. Sometimes they're waiting for the hiring manager to reply to an email. Sometimes they received hundreds of applications for the position and are working their way through them before calling people back.
In any case, the reason for the delay shouldn't affect your behavior. You've called back already. Give it a few days. If you don't hear back, it may make sense to call again in a few days. You should always operate on the assumption that they're not going to call back/ offer you the job/ etc. so you should be applying to other opportunities.