Hi Jenn! Welcome!
Okay, I think I see what your saying. You're getting a ladder up the middle of the blanket, where the stockinette section meets the reverse-stockinette section?
I think I remember someone else talking about that somewhere, so you're not alone. Is it possible that you are not bringing the yarn to the front, first, to purl, and to the back, first, to knit? Or not approaching the stitch from behind the working yarn, to purl, or in front of the working yarn, to knit? Check out the videos on working ribbing, to see how to alternate from knit to purl....
Otherwise, I can't imagine how a ladder would be happening. Normally, you don't need to work extra tight, or anything, and if you're working really tight at that join, I can only think that you are somehow wrapping the yarn over the needle in an extra way.
Hope that helps!
KnittingHelp Queen Bee
It is not because things are so difficult that we do not dare; it is because we do not dare that they are difficult.