My grower is also corn and soy free. I wouldn't worry too much about trying to fatten them up unless they're actually losing weight.
With whole grain feeds it's best to serve them fermented or at least wet, to get all the components clumped together to prevent birds from picking out favorite...
Kinda confused as to why they'd be getting thin from layer feed?
Layer does generally have lower protein, so I feed both layer and grower. No it won't cause egg laying issues, but you should have oyster shell on the side so layers can increase their calcium intake as needed.