I’m trying to make the big block afghan. I frogged once because of this looseness problem. Every time I start a set of purls…the first stitch ends up extremely loose. When I wrap the yarn around and start to put my needle through the stitch on the left needle, I can see that stitch getting all loose and pulled. I have been pulling really tight when I start a set of purls, but still no success 
I sooo wanted to make this damn thing. Any other suggestions…here’s a pic. The yarn looks grey, but it’s really hunter green.
Are you using circs or straights? It might help if you use 2 different size needles… a smaller needle for the purl rows. 
Circs
I kinda had this problem when making my mother’s afghan. I was using size 17 circs. My sts tended to stretch/pull when they were on the cable. What helped me was making sure to keep the stitches tightly bunched together when they were on the cable. Hope that helps a little…