It is possible to have some dirt under the sealing surface or even in the Gasket Groove if you did not clean the Head and Cover off before removing it.

It is also not unknow for the Valve cover to develop a hair line crack or even be warpped.

I also suppose it is possible that the prior owner had overtightend the nuts and squished out the Aluminum and not you cannot tighten it down as well.

Even when I install a new Valve Cover Gasket I go to the local 99 Cent store and get a tube of Silicone Sealant.
I put a thin bead inside of the Gasket Groove; also I degrease the bottom edges of the valve cover.
After the Gasket is on the Valve Cover I put a thin layer of Silicon Sealant around the bottom of the Gasket.

I wipe off all the Oil and dirt from the Cylinder Head and install the Valve Cover. I only slightly snug down the Nuts and allow the Silicone Sealant to cure several hours; after that I tighten it for good.

When I re-use the Valve Cover Gasket I just rub off the old Silicone Sealant on the bottom edge and renew with new Silicone Sealant.

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84 300D, 82 Volvo 244Gl Diesel