MANY folks (including me) will tweak the lips (bottom portion of the cover) wider to avoid scraping the upper portion of the receiver. If the lips of your new dustcover are tighter? Not much surprise.
A new dust cover can be adjusted by noting the tightest area (usually toward the very rear), PADDING a pair of pliers, then gently bending outward on the lip - each side about the same. Work around the entire lip area as you see a new ultra-tight spot, and don't make it TOO loose -go slowly.