Hey there,
How wonderfull - your frog friends are comming of age..
Yes it is to be expected that the calling of one will often cause the others to join in - In fact I use this to try to stack the odds when I buy a new frog - I take recorded craoking to the shop and play it to the prospective frogs - any which start croaking don't come home with me as I have enough males (5) - Sometimes the females will also have a go at craoking (don't let anyone tell you they don't croak) but females usually only craok once or twice - or simply quiver in rythm with the croaks - anny which stand up to they're full height and really go for it are male most certain - It's not foolproof though as young frogs don't croak male or female
It's been my experience that the frogs need to be arround a year old before croaking with any enthusiasm.
Keep an eye on your silent frog whilst the other is croaking - look for any rythmic contractions in time with the sounds - you might have been lucky first time round - but you will still need more males if you want to breed as the females are fussy about who they let on.
Hope this helps