what could yellowing leaves mean?

[inkeys]

Hi everyone, I am new here.

I have been keeping orchids for about 2 years now, and I have acquired seven of them. I have not had the greatest success with them. The only one I have gotten to rebloom is a phalaenopsis i inherited from my Mom.

Recently I have noticed the leaves on some of my phalaenopsis are turning yellow and two leaves fell off one of them today, with another leaf looking very soft and not healthy, so that will most likely be the third to drop.

I know it is normal for orchids to drop old leaves, but is three at once too much?!

they are all kept in a bark mixture and watered once a week, in a southern exposure behind blinds.