Before deciding that your dog is being disobedient or willful, ask yourself the following…
Has the dog really had enough time to get the behavior down pat?
Has the dog reliably performed the behavior in many different places and in many different circumstances?
Has the dog possibly been responding to cues other than the ones you intended?
Is there something new in the dog’s environment? Have you brought a new dog home? Have you a new houseguest?
Is it possible that you’ve worn out the cue by overusing it before the dog really knows its meaning?