A common mistake many people make when setting up a queue is not to add the line autofill=yes to the queue.conf entry thsi will mean that all callers will queue until a call is answered.

See the note here about the behaviour of the queue with it set to yes and if its left out or set to no

autofill - (default value - yes) - The old/current behavior of the queue has a serial type behavior in that the queue will make all waiting callers wait in the queue even if there is more than one available member ready to take calls until the head caller is connected with the member they were trying to get to. The next waiting caller in line then becomes the head caller, and they are then connected with the next available member and all available members and waiting callers waits while this happens. The new behavior, enabled by setting autofill=yes makes sure that when the waiting callers are connecting with available members in a parallel fashion until there are no more available members or no more waiting callers. This is probably more along the lines of how a queue should work and in most cases, you will want to enable this behavior. If you do not specify or comment out this option, it will default to no to keep backward compatibility with the old behavior.

So you can see by not setting it which is the default in most cases all calls  will queue even if you have free agents waiting to take calls.