I just replaced mine two hours ago. When I removed the old one the screws didn’t seem to be too tight. The paint under the original petcock appeared to be degrading so I cleaned the area with some scotchbrite. It looks like they may have had some type of sealant applied at the factory when it was originally installed so I wanted to remove that. I had to take a Stanley knife and trim off the old fiber washers under the screw heads from the old petcock because I didn’t get any new ones with the new petcock, they were stuck to it. I installed it with a firm application of a large Phillips screwdriver, but nothing extraordinary. It doesn’t appear to be leaking so far.
You may have a rough surface or something under the rubber seal. Try smoothing the tank surface with some Scotchbrite.