Howdy...
It's likely your water is too high or low pH.
Do you have a meter or a pH kit?......Yer gonna need one if ya don't have one.
the liquid ones are dirt cheap. last awhile, and will be accurate enough for ya for right now.
You can take a sample of your tap water to your friendly neighborhood hydro store, and ask them test it and hook you up with some pH solution according to what your water tests out at. it might need pH up, or down...we don't know.....YET!!!
But they can tell you, and they'll sell you what you need to correct it.
Are you using any nutrients or root enhancer, or just tap water?
You are gonna want to use 1/4 strength DWC nutes (whatever kind you like to use) and add some root starter to your DWC water....I use "rapid start"......and keep that pH down!!!.....around 5.3 to 5.6 is just fine......check it often!......especially if it is a small DWC reservoir.
Also, I've had bad luck with DWC cloners coz they tend to keep the coco or rockwool too wet....it hinders root production if it's too wet.
When you don't get root production after about 10 days, the cuttings start to tap into their own reserves and essentially eat themselves because they haven't got the roots to absorb food with, and are starving. If it's too wet, that means the cuttings aren't stimulated enough to send out roots to FIND moisture....if moisture is already there in abundance..... Then they get "lazy" and starve.
I did some experimentation with lowering the water level in the DWC cloner, it helped...but it didn't solve that problem.
I ended up going back to coco/rockwool and a clone dome again.....it worked waaaay better.
I hope this helps!!!