Cast on doesn't go all the way around when circular knitting

Okay…I’ve never knit using circular needles. I have a patter for a diaper soaker that is for knitting in the round. It has directions for the different sizes, s,m,l. It says to use 16"in circulars size 6. But if I’m doing a newborn or small size it doesn’t fit end to end. I’ve watched the videos here and she is only showing how to do when her ends meet. Is it possible to still do it? I’m totally confused. Is there other directions online somewhere that can show me? Or will have to use double pointed needles to get the small size?

The cast on edge is tight before knitting the first row anyway. It’s not uncommon for it to be a bit of a stretch to get those stitches all the way around. But after a few rows, everything fits comfortably.

However, if the gap between the start and end is bigger than a half inch, even with stretching the stitches as much as you can, it’s just not going to fit enough to knit the first round.

You could cast on using a larger needle size, that will give you more room to stretch them around, or you may have to use DPNs for the first couple rounds. I’d try a larger needle first, but remember to switch back to the smaller size before knitting round 1.