Well, we can run through the laundry list:

- Overcrowding/competition
- Have you fed them at all? Liquid fertilizer?
- Dry compacted peat in soil mix; difficult to re-wet
- Salts build-up in the soil; no flushing
- Overwater/underwater? Doesn't sound like it, but..
- Too much sun/not enough sun

Post a picture and tell us about your soil mix for further diagnosis.

NGI are a very thirsty lot. I've got two and if I forget to water every other day they look absolutely dead and wilted but after watering they spring back to life within a few hours like nothing ever happened. It does however stress them so they flower less.

Just a guess but from my perspective I would check to see if they soil is hydrophobic at their roots using a bamboo skewer or skinny wood dowel. Your watering may be bypassing some of the roots. The potatoes and some begonias are thirsty plants. (not sure about coleus?) The potatoes are very agressive.