What was the last amendment to be ratified by Congress but not ratified by enough states?

The proposed Constitutional Amendment most recently approved by Congress as required is the Washington DC Voting Rights Amendment. It would have have given the District of Columbia the same rights as a state regarding voting representation in the U. S. Senate and House of Representatives and regarding Presidential elections. Congress approved the proposal in August 1978. By the time it passed a self-imposed expiration date in 1985 only 16 of the required 38 states had ratified it.