Hey Sand. They were designed to be hard to remove. Once the rubber gasket it gone, they come right out, though. I have a long post about stoppers on here somewhere. I'll post a link if I can find it.
The link...https://www.antique-bottles.net/forum/m199654-m199654.aspx#199827The stopper is thicker and wider at the bottom-most point. That makes it harder to remove with the gasket on it. The metal ring just above is thinner so that the rubber gasket can fold upward during the installation of this seal. Once it is all the way in, the gasket will prevent any leakage.