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I think putting somebody on Hold still counts as being "busy". The call queue interprets this as the user being unavailable, so it doesn't automatically offer the call that is waiting.

I can think of 2 ways to go about it, although I haven't tested them
1) Use call queue pickup. Groups > Call queues > Click on the call queue > Pickup settings & members

2) Park the active call. This will change your status to available, and the next call in queue will ring automatically (assuming you've set the wrap-up time to 0 seconds). For this you must configure park locations under Groups > Park Locations.