Had a loss of power from my Marta's after playing in the snow recently. Tried bleeding several times and still lost power after a bit of riding. It loked like a bit of oil was leaking from the levers...
Phoned up Magura/sent the brakes off - 2 days later they came back working like new ~ not quite sure what they did as the service info was in German!
5 year warranty worth it's weight I'd say - top service!
If you could see oil leaking from the levers and you have the older style levers then that's what happened to one of mine. Not serviceable, needs a new lever assembly. That's when the 5 year warranty would have been good, unfortunately I bought mine as an ex display pair off ebay from someone in Spain. The Magura service department were still very helpful though, and sorted me out with a replacement at a very fair price.