Do you repot them after you get them? For Phals, I try to repot as soon as I can, with or without blooms, oftentimes the roots in those containers are just so suffocated. So by the time the plant finishes blooming, which is really a long time for them, collar rotting has ensued and it dies. That was one of my mistakes before, I waited too long.
That's for Phals only, for other orchids, I have to wait if they are in bloom when I acquire them before I attempt repot.
