You should at least have some roots coming out of the bottom into the water now. I have them in cups as you would start an avocado. After about 2 weeks or so the roots started to form, and this is where I am now. I have the cups in a sunny window. I wouldn't be overly concerned, just keep watching them.

Thanks so much for all the advice and information grandad, toogreen, farmerdilla and wayne. It sounds easy indeed, now I wonder why we did not try this before. In the worst heat of the summer, only the okra looked good last year. I cannot believe that plant, flowering when it is 107 F!!!! crazy plant (I love it). This year we are adding cow peas and sweet potatoes to the hot weather vegetable clan. Why not? might as well take advantage of what grows well here and now. We cannot grow parsnips or cherries, so we might as well grow sweet potatoes. We have several roots (tubers?) starting, I only showed one, but we have white sweet potato and red sweet potatoes. They have different types of leaves, as shown in Farmerdilla's picture. Farmerdilla, your picture is nice, thanks for posting it. We might do a bed just like that, still have a few recycled pieces of lumber to use for garden beds. I think that 500 slips is a bit much for us, we might do from 12 to 20 slips. Since each averages about 6 sweet potatoes, this should give us plenty for feeding two not too large people. Besides, we are also growing other 'starchy' root crops, regular potatoes (to be harvested soon), jerusalem artichokes, chayote (the root is edible) and some weird tropical ones (elephant ears types). I take it that the level of development shown in the pictures I posted is just about right for taking the slips? not too large but large enough? or perhaps they could be 'harvested' as slips a little younger? We still have them in water but we will plant them in the ground, or in pots, this coming weekend.

My guess would be multiple nutrient deficiencies, particularly that of phosphorus. Have you added any fertilizes or amendments to this soil apart from the bulk compost? Commercial compost can vary widely in quality, and if the stuff originated from woody materials, it could be low in nutrients. Try applying a balanced fertilizer with micronutrients, either organic or synthetic. Synthetic will work faster.

I am a preschool teacher, each year I have my class start a sweet potato plant by placing it in water in the classroom. It will take a few days for it to root then in about a week it will start growing the vine. The vine will grow very fast! When spring comes we plant it in our flower garden. The vine will spread a very long distance. We dig up our sweet potato plant close to Thanksgiving. Last year our one potato produced 25 potatoes of varying sizes. We found several recipes to cook up for lunch. I am sorry I don't know how to attach photos, my students always have fun with this project. We will have to try a sweet potato leaf salad now!