I have the same problem...

Too much nitrogen maybe. Would be great for many crops… A lot of mulch?? You have to deprive nitrogen more to develop roots… Maybe not your problem but I have had this one

Plant them into a row of greenhouse plastic. They love the soil to be hot🔥

I had a few that would've been nicer but the voles got them. If I can line the bed with hardware wire and build a greenhouse tent over it I might try again.

Those look pretty good. Did you start from just one? When I plant sweet potatoes, I use a bunch of starters. They don't have to be big or spread out much, just grow more than one. A 3'x3' bed gave me more than 30 lbs starting from about 8 pieces.

try black plastic mulch. they need heat.