Just to check - the leaves that have dropped, are they from the bottom or from the top? It is quite natural for the leaves to yellow and die, dropping off from the bottom as the plant grows. This is the way that phals grow, and you will gradually see roots emerging from the lower regions of the plants body (don't know the technical name) as there are more leaves higher up to support it. All phals seem to have a different number of leaves that they need. I have a couple who seem happy with 3 leaves, and I then there are two I can see at this moment that have 8 and 9 respectively. The one with 9 is quite a small plant, the one with 8 is massive.
If the leaves have dropped from the top, there is porbably a problem, and I would suggest that you post a couple of photos to allow forum members to look at the plant and tell you what they think is wrong.
If it's just the plant naturally getting rid of it's older leaves, then it is likely that once the plant has flowered it may send up a new leaf from the top as well (or like a couple of mine do it all at the same time and waste energy!)
hope this helps,