Backslide and Fall flat

Backslide verb - Drop to a lower level, as in one's morals or standards.

Fall flat and backslide are semantically related In some cases you can use "Fall flat" instead a verb "Backslide".

Fall flat verb - Fail utterly; collapse.

Backslide and fall flat are semantically related in be unsuccessful topic. Sometimes you can use "Backslide" instead a verb phrase "Fall flat", if it concerns topics such as fail to attain goal.