Hi mate apologies about the brake, it was working tip top for me. however i do know what has happened, in fact it has happened to 3 of my previous maggies, completely at random. As said above pop the piston out and give it a good clean and lube, pop it back in and it should be fine again. If that doesn't sort it, pop the piston out again and sand a tiny bit of the piston and keep refitting until its sorted. If that doesnt work, you can send it back to me and il sort it out for you.
Sorry again.
EDIT - beaten too it with the solution