They are having severe nitrogen deficiency. When organic matter start decomposing, the bacteria use all the available nitrogen. As they die they will release the nitrogen. This can take a couple of weeks to a couple of months. I would use a high nitrogen fertilizer and help balance out the mix. It is a common problem when putting in a new bed. By next spring it should be doing well. The only way to correct it is to use a high nitrogen fertilizer or give it time to decompose.

Just a thought is there any signs it might be a spider mite attack? Webbing, gritty feeling on the undersides of the leaves?

Thank you both. Gary - what would you recommend? I've been using fish emulsion, is that enough?

One of the problems with using organic fertilizers in this situation is it too will need bacteria to break it down. Fish emulsion has about 2% nitrogen, where most synthetic fertilizer have much more. Ammonium sulfate has 21-0-0 which means it is 21% nitrogen without the phosphorous and potash that you probably do not need. With organic gardening you usually have to anticipate the problems you may have. Some organic fertilizers such as chicken litter may have a faster effect than the fish emulsion. But it will be adding some nutrients that you may not need.