Great to see your polybook 🙂 , you have some nice work in here!
For me , the biggest issue is lighting. I feel that you rmodels do not “pop” because the lighting is very flat and uniform.
Have a look at the info on the Blender market for Andrew Price’s Pro lighting Studio here :
https://www.blendermarket.com/products/pro-lighting-studio
In the text Andrew describes how lighting can really make or break a piece .
Also , these may help you :
https://cgcookie.com/course/fundamentals-of-lighting
https://cgcookie.com/t/538-match-your-favorite-lighting-example
