Can someone tell me what I did wrong here (photo)?

I make this mistake every now and again (see photo) usually by the time I notice it I've gone several rows. I've tried fixing it as you would a drop stitch (dropping the stitches down to the affected row then picking them back up) which obviously it doesn't work or I wouldn't be asking lol. It's more noticeable on the purl side (photo) ob the the knit side the stitches appear slightly larger than the rest.

Can someone tell me what I've done wrong and how to fix it?