You may need to use a little hardener to get a stiffer plastic. It doesn't take much, but as rut mentioned above, softer plastic seems to cause the bait to tumble through the water column where a firmer plastic swims a little straighter allowing the boot style tail to kick and the body will have the slight undulating rolling action you're looking for. Also, try using a smaller hook in your swim bait. If you use too big of a hook you can also have some issues with the bait rolling over and acting funky in the water. On a 5" swim bait I normally use a 4/0 EWG weighted hook that has the screw lock keeper. Owner and Gamakatsu both offer great hooks in that style as do others, those are just my two preferred with the Owner hook even having an adjustable weight on the shank to alter how your bait swims in the water.