5 Month old Pup, seems his poo is a little harder to get out these days

About a month ago we started Marley off on Nature's Variety Instinct: Duck (dry) and instict Raw: Lamb. The lamb he gets every few days but im considering supplementing it more in his diet. Recently he seems to struggle a little more getting them out. He's not constipated as he still goes, but when he does the stool seems much firmer than when on his Blue Buffalo Chicken (dry). Is this a result of poor food choice? Or is it normal?

Thanks in advance I truly appreciate the help!
Josh

Any dry food will result in firmer stools.

I don't know how good Nature's Variety is because I feed raw, but maybe someone else here has experience of it.