It could be that your trigger assembly is correct but the inside of the cylinder opposite the trigger assembly has been worn by the piston tail.
This allows the piston to move upwards and slide forward over the sear as if the sear is not engaging with the piston.
My first Meteor had this problem but the previous owner had fixed it by hitting the top of the cylinder down with a lumphammer and blunt chisel.
I've still got the gun and it still works so I wasn't robbed of my £2 (1971 prices eh).
All the best Mick