I've read only 14 hours, and I bet most will lay with 12 hours of light. It can be just a 25 watt red bulb as well.
Have you tried supplementing their diet with a source of lean protein? I find canned sardines or tuna help to speed a molt, but if you have a lot of chickens it can be expensive.
How old are your hens? It takes longer to re-grow feathers and complete a molt the older the hen.
What about an environmental conditions - such as too little ventilation in the coop or nighttime noises which may be disturbing their sleep?
Just trying to think of anything which might help you.
Have you tried supplementing their diet with a source of lean protein? I find canned sardines or tuna help to speed a molt, but if you have a lot of chickens it can be expensive.
How old are your hens? It takes longer to re-grow feathers and complete a molt the older the hen.
What about an environmental conditions - such as too little ventilation in the coop or nighttime noises which may be disturbing their sleep?
Just trying to think of anything which might help you.