I don't grow dendrobiums but for cattleyas, the sheath will form when the psuedobulb first grows out. If there's no sheath it won't grow one later. But spikes sometimes form on psuedobulbs that don't have sheaths. Just because a psuedobulb has a sheath does not mean that blooming in immanent. It can be months before buds form in the sheath and you'll always get some sheaths that buds never form in. The best indication of when a cattleya will bloom is the time of year. I keep a record of when each catt blooms so I have an idea when to look for new buds. Good luck.