The two most common problems with cedar roofs are: they aren’t ventilated properly and thus rot and have to be replaced, or the cedar roof wasn’t installed properly and tiles keep breaking or falling off.
On an installation level, the most common mistake made is lack of a stainless steel ring shank nail being used to install the cedar shake roof tiles. A regular nail won’t hold, because it’s a smooth shaft and, combined with a really soft wood like cedar, never holds. The ring shank is needed to grab and bite the soft cedar wood, almost like a screw would thread and hold fast to it. The stainless steel part is necessary because a regular nail will corrode as it is exposed to the elements on a daily basis. The shaft will deteriorate to almost nothing and snap, causing the tiles and brackets to fall off.
If this is your problem, there is little that can be done except re-nailing the entire roof with stainless steel ring shank nails. If ventilation, or lack thereof, is the problem, again there is little you can do without tearing off the whole roof and starting over.