Stuart, the ring about 3 or 4" in diameter that holds the lid on the cup?
There is a hard cup and a soft liner and a hard cover with filter and the aforementioned ring. Right??
Insert the soft liner, insert the hard cover and screw on the ring? Check to be sure you have the soft liner and the cover inserted fully. There is some friction. Wipe the threads. Run a fingernail around both of them checking for burrs.
What have we missed?