So you are using the following settings then?
Max Characters 15
Minimum Characters: 4
Max Alpha: 0
Min Alpha: 0
Max Numbers: 0
Min Numbers: 0
Max Special: 0
Min Special: 0
by setting Max Alpha (In other words maximum number of letters allowed), Max Numbers, and Max Special to 0, you are telling it to not allow any of the characters found on the key board to be entered.
you are overriding the min/max characters settings by setting all of the other max settings to 0, leave them blank instead of using 0.