For some reason I can't post a picture of this type flapper valve. It has two cylinders at right angles to each other and a screw-on beveled green flat plate that actually does the sealing. Some have the plate just snap in.
Anyway, the 3/4" lg machine screw that holds the flat plate seems to have enough play in its sleeve so that the plate moved 1/8" closer to the flapper hinge and so prevented its closing. This happens infrequently.
Has anyone had this problem before? I'd get a whole new assembly if I were sure the new one wouldn't have the same infrequent problem.