'Fullness' Not Full

If the plates located for Joseph Smith by Maroni had the fullness of the everlasting gospel contained in them (D&C 20:8-9; 27:5), how is it that Mr. Smith added new revelation to the gospel, and with very strong language? "For behold, I reveal unto you a new and everlasting covenant; and if ye abide not that covenant, then are ye damned; for no one can reject this covenant and be permitted to enter into my glory." D&C 132:4-5. Either the original plates did not contain the fullness of the gospel or later prophesy was spurious...or BOTH are lies!