I have one of these; bought 2 years or so ago, second blooming. It seems that every season, as one pseudobulb matures, one of the others degenerates and eventually just falls off. I haven't had any problems with the plant but I'm curious if this is normal and I will forever have a plant with no more than 2 fully matured backbulbs?
That is indeed how they grow. The old pseudobulbs tend to wither and die, not always in a year, but quite soon. If it is grown well it will sometimes starting putting out two new growths instead of one, and eventually you'll have a bigger plant, but if the pseudobulbs are as large and fat as the previous then you're doing things correctly.