Atco Balmoral 20s carburettor seal not fitting in its groove?

hi i recently cleaned my carburettor on my Atco Balmoral 20s lawn mower as it seemed to be dripping fuel from its carb, and now i can not get the seal (like a rubber band) to fit in its groove, keeps popping out. not sure if it needs replacing. anyone else had this prob or any advice on how to get it to fit in again...

i think this may have caused the initial leak.

thank you, luke