Are you sure this part is supposed to be knit in the round? If so, you probably need to cast on again, even looser. You can hold two needles together and cast onto them together if need be. Remove the second needle before joining the round.
Also, joining the first round is sometimes quite a stretch. But once you get that first row knitted, it will loosen up and fit nicely around the circle.
If you just can't get it to reach all the way around, no matter what you do, consider getting a 16" needle instead.