Half of baby blanket is loose!

I’m fairly new to knitting but I’m half way through a baby blanket.
The pattern requires you to start from a corner and add a stitch every row then when reached desired length to then decrease a stitch every row, however on the decrease rows it’s a lot looser and ends up out of shape!
On the decrease rows I have to do 2 knit 2 togethers which is causing the problem and I don’t know how to correct it.
Can anyone help?

Do you have a link to the pattern? Don’t post it here. Just give a link or at least name.