My toddler is 18 months, has about 30 words. Can say moo, milk, more etc so M's aren't the problem. Says daddy a million times a day, in the right context I.e to his dad "up daddy/no daddy/yes daddy" but if you ever, and I mean ever say "say mummy, say mama, copy me, mama" he just says "No, DADDY!"
He did it a few times in front of people who laughed a lot and found it hilarious, so I think he may just be performing now. But it sucks! Will he ever say mummy? 