Bought the OGGI 7209 Penguin Cocktail Shaker SS last time it was one here. The damn thing is still on the counter sealed shut, because no one can get it open. Once you put it together, don’t expect it to ever come apart again.
A cocktail shaker makes a slight vacuum when you shake something with ice. Be careful not to close the tins too tightly or, well, that happens.
You can try running hot water over it. Might help.
Tho, if it’s a while ago, you should be able to take it apart.