Neglectful, a. — Webster enters it under “neglectful”
Full of neglect; heedless; careless; negligent; inattentive; indifferent. Pope. A cold and neglectful countenance. Locke. Though the Romans had no great genius for trade, yet they were not entirely neglectful of it. Arbuthnot. -- Neg*lect"ful*ly, adv. -- Neg*lect"ful*ness, n.
Shades of meaning
Fernald has no entry of its own for neglectfulness; he lists it under neglect.
He lists it without comment; the entries above explain its neighbors.