Function Words

There are a bunch of verbs which end in "ect" (affect, defect, effect, infect, perfect, deflect, inflect, reflect, neglect, abject, object, project, correct, direct, detect, protect, respect, etc.) that can be turned into adjectives by adding either the suffix -ive or -ful. The vast majority of these "ect" verbs use -ive to make adjectives but "neglect" requires -ful (neglectful but not *neglective) and "respect" can use both -ive and -ful (respectful and also respective). The two adjectives 'respective' and 'respectful' have clearly different uses.
What is going on here?
It may be time to investigate the English adjective-making suffixes a little more deeply.