This worked for me:
var tileURL = 'http://{s}.tile.cloudmade.com/{api-key}/1714' + (L.Browser.retina? '@2x': '') + '/256/{z}/{x}/{y}.png';
L.tileLayer(tileURL, {detectRetina: true}).addTo(yourMap);
Have you tried with different tile styles and check if that might be the issue?