I think it is most likely something to do with the very low temperatures we've been having recently. Even though you have started to harden them off, maybe the soil hasn't really warmed up. I guess your options are: 1- put some sort of protection over them (eg cloches) and see if they pick up - you can take these off during the day, but pop them back on at night until we get some warmer weather. 2) Repot them - but I fear that might be too stressful for them 3) hope they will recover independently. I've had cloches over the spaces where I'm planning to plant out my courgettes for about 6 weeks now, and when I eventually plant them out, will continue to protect them as in 1).