I seem to totally suck at raising diffenbachias. Both of mine suffer from the same problem, which is that the bottom leaves dry and curl up, and eventually fall off. It starts with the tips of the leaves turning crispy, before the whole thing goes yellow and drops off. I've experimented with light levels, watering levels, spraying every day vs. not, and they just don't seem to enjoy my condo. Our place is angled north-west, so it gets a moderate amount of light, and a bit of direct sunlight later in the afternoon. It can get fairly dry in the winter, but my other humidity-loving plants don't seem to mind too much. Any help would be appreciated.
normal growth pattern for diff's is that new growth appears at the top and the older growth will die off after a time. sounds like your plant is doing just fine if that's what is going on. now, if the new growth at the top is browning up, then there's some problem...if this is the case, please post some pics of the plant and close-ups of the problem area. it's so much easier to id a problem when you can see it :)