Neckband cast off too tight

I have cast off a neckband of a sweater using a size bigger than the called for needles so that the neck would have a little give and not be too tight. Unfortunately, that wasn’t the case. I have tried using different size needles (bigger and smaller) and I’m having no luck. The pattern called for a size 5 16" circular needle. I would call myself and intermediate/experienced knitter and have never encountered this problem. Any suggestions? Thanks!

Without the neckband, would the sweater neckline be too tight? You might have a smaller gauge, so it’s too small overall. Or you just might need to go up 2 or 3 sizes for casting off. Usually just one size up won’t be enough.

Have you tried using a crochet hook bo?

I will have to try those suggestions. I might just go back to picking up stitches for the neckband and try again. Thanks for the tips!