Looks pretty good man. 24/7 light won't hurt them any, it's all about preference (some growers think it speeds up the veg process). The rock wool cubes look a little damp which is fine but I've had some gnarly mold grow on mine from being wet and humid for a long period of time.

As far as two doing good and 2 doing bad...ehhhhh, they all grow a little different and at different paces.

I'd say one more light for now wouldn't hurt and just keep checking on them to see what happens.

I've got a similar set-up you can check out and ask more questions if you need to. Every set up is different but, weed is called weed for a reason...shit will grow damn near anywhere lol

Happy growing bud :thumbsup: