Hmmmm. Firstly I'd get you to reflect on it. Do you think there is any truth to the comment? If there is, obviously work on it.
What's your support like? Who is your line manager? Are there other Drama teachers in the dept., or are you the only one? It would be helpful, as a first step, to see what everybody else is doing. Could you perhaps ask to be observed by somebody you trust and like and get some feedback? Or maybe you could observe some practical lessons yourself?
I am a former Head of Drama, but am mainly an English teacher, really. I actually found Drama significantly easier to differentiate than English, just because so many of the tasks are skills-based and open ended. I know you're not supposed to 'differentiate by outcome' explicitly, but actually, Drama definitely does.
I'm sure you're doing all this stuff already, but do you have clear learning objectives in your lessons? And are you making a point of returning to those at the end of lessons to check they've been met?
Are you obviously differentiating, making a point of having a range of tasks for different abilities? Organising groups by ability, that sort of thing?
Coolpaper, I'm sure you're doing all that stuff already....maybe just make it more obvious? I'd also ask the head which class it was, as a start, and definitely make a point of concentrating hard on the lessons for that group in particular.
Alternatively, if you think it is the content that is the problem, there are always theory lessons, which you could give (though those may cause the rest of the class to complain!)?