the novels Order 66 and Imperial Commando: 501st partly deal with a transistion away from Kamino-produced clones and increased numbers of mongrel (IE baseline human) officers in the forces. it's also made clear that combat aside, clones have a limited life-expectancy due to the Kaminoans accelerated aging (something like 20 years in 5 calender years) of them to physical maturity.
so, by the time of ANH, nearly all the clones from the Clone Wars are dead either through combat or through aging. thus, the Imperial Army and Navy are instead reliant on either conscripts or volunteers from the human populace. and since they didn't spend the first five years of their lives being drilled in tactics, combat and live-fire exercises, they aren't as good.