If they are brown and crispy i would think the soil's too dry.
Dry and soft and it's too wet.
Alternatively, it could be that the foliage has got wet, had the sun on it, and scorched it.

my first thought was too much sun , they seem to do better in dappled shade. however one of mine has a reddy hue to the leaf and from a distance it looks brown. any chance of a photo?