Signs of an overwatered vanda

[BradGC]

I have a small collection of vandas and although most of them grow happily, I have about 2 or 3 that are struggling a bit.
I've noticed that it's usually the ones planted in plastic pots as opposed to open baskets and terracotta with holes.

I'm assuming that I might be over watering them (I water them all everyday but perhaps these still have moisture in the pots.
Basically the bottom leaves are slowly dropping. Could this happen from over watering?

I used to think that the only bad that could come of over watering would be root rot but I always keep an eye on the roots.
Usually I plant them straight away in terracotta with holes, but if they have come.from.a nursery in a pot, I like to leave it in there as I don't want to upset the roots.