I have a problem fitting the Welsh/Core/freeze plug on the side of the block, it is between the Cam belt and Tensioner, I bought a new plug for replacing and the dimensions seem to match, the old one came off without much trouble, however Upon fitting the new plug I tried using the old one as a guide to knock the new one in, I went further by tapping it on the sides to try make a solid seal but there is still a leak and this plug is able to pop off under minimal pressure.

I went further to tap the inner dome on the plug in the attempt to create a flair in hopes it would seal, but to no avail. I what am I doing wrong and is there a specific method of installing this?