Connecting sections of a dog sweater

New knitter here. Making Ozzie dog sweater, have a strip that goes between his legs and connects to the upper part. The strip that goes between his legs is on a stitch holder with one working yarn on the end and now It’s time for me to connect it. Pattern says put all stitches on one needle and continue working the pattern in the round for 20 rows. I now have all stitches on the needle but when I get to the part where I will connect the upper part to the lower, it’s too loose. Suggestions? Thanks for any help!

Link to picture of finished project Ravelry: The Ozzie large dog sweater pattern by Jenna Greer

Welcome!
Many thanks for the pattern information and link.
Too loose in the sense that the stitches are loose at the join? Can you snug up the joining sts by pulling on the yarn tail? That join should become more solid as you work more rows. Or is it something else that seems wrong?