I'ts not the gaskets, it's the way the are being installed or bent or cheap chrome valve covers.
The way I do gaskets on my valve covers (and oil pan and timing cover etc.) I use cork gaskets, put gasgacinch on the cover (make sure the cover is clean, real clean) and one side of the gasket, then attach them to each other, this keeps the gasket on the cover. On the head side if you have a good flat surface, like an aluminum head, I install them dry. If cast heads and a rough surface, put a thin (approx 1/8") layer of rtv on the gasket (head side), let it sit just long enough to form a skin, then put the cover on. I have yet to get a leak from either method, I have cork gaskets on my factory aluminum covers that have been there for 4 years at least, and I have them off a couple times a year.
When you put them on DO NOT OVERTIGHTEN, thats the biggest problem. The wise old Mechanic that taught me this said use a 1/4" drive or "choke up" on a 3/8" drive, so you are not getting hamfisted when tightening.
But, if your cover is bent or cheap chrome, all bets are off. Get a good steel cover or better yet aluminum, aluminum IMO are the best because they don't distort.