Synonyms for Failing:

adj.

feeble (adjective)
fragile.
losing (adjective)
losing.
not well, weak (adjective)
unsuccessful, faint, feeble, declining.

n.

aging, imperfection, flaw, defect, feeble, Achilles' Heel, faint, inadequacy, deficiency, dying, weak. barring, fault, frailty, Into, better, aboard, foible, Amid, From, Among, at, shortcoming, help, infirmity, Against, weak point. frail, worse, sick, ill, unfit, unhealthy, infectious, unwell, fragile. goner, someone's/something's days are numbered, fatally, not long for this world, someone's number is up, on your last legs, at death's door, on your deathbed. failing (noun)
deteriorating, regressive, regressing, unsatisfactory, weakness, unsuccessful, retrogressive, declining, retrograde, failed.
lapse, shortcoming (noun)
deficiency, foible, flaw, infirmity, weakness, weak point, imperfection, defect, frailty, fault.
state (noun)
weakness.
weakness (noun)
weak point, foible.

v.

failing (verb)
faltering, Blundering, stumbling, Flunking, busting, Collapsing, Floundering, falling, Flopping, losing.
neglecting (verb)
Neglecting, Forgetting, disregarding, Ignoring, overlooking.

Other synonyms:

inadequacy, Into, Against, frail, goner, Among, barring, imperfection, unwell, deficiency, From, aging, weak point, feeble, unfit, Amid, Achilles' Heel, dying, unhealthy, worse. at, frailty, aboard, fragile, fault, fatally, defect. better. ill, shortcoming, sick. infirmity. unsatisfactory
unsatisfactory.
Other relevant words:
aboard, Against, fragile, From, unfit, goner, deteriorating, fault, retrogressive, shortcoming, better, imperfection, regressing, ill, unhealthy, sick, failed, unsuccessful, inadequacy, fatally, Into, flaw, worse, infirmity, frailty, dying, retrograde, regressive, weakness, weak, frail, aging, foible, feeble, Achilles' Heel, unwell, weak point, unsatisfactory, deficiency, declining, Amid, at death's door, defect, Among, barring.

Usage examples for failing

  1. " I will deliver her to you safe and sound," said he, " and, failing that, I will make her queen." – The Prisoner of Zenda by Anthony Hope
  2. Rapidly failing as he was, I was certain that he was by no means without power of speech. – The Best Short Stories of 1921 and the Yearbook of the American Short Story by Various
  3. But then, we learn to expect failure; and, most important of all, we learn to succeed while failing – The Boss of Little Arcady by Harry Leon Wilson