I've had the exact same problem when I was designing (trying to) websites. It's the same but 2D and simpler. I was feeling the result is fine but quite not right. Compared to "real" website made by professional, my design was clearly inferior but I could not fanthom why exactly.

Only later I discovered it was the font size few pts smaller, the padding 1 pixel larger, the color little bit less contrasting, the 1px additional light border on top of dark border, the shadow a bit more blurry and million subtle things you can get right and balanced only with experience.