Look at the different way to express ''not enough'':
Not + [adjective / adverb] + enough = ne ... pas assez + [adjectif / adverbe]
Note that "assez" is always placed before the adjective / adverb it applies to (unlike in English).
Not enough (of) + [noun] = ne ... pas assez de/d' + [noun]
Note also that "de" in front of nouns never becomes du, de la, de l' or des.
It will however become d' in front of a vowel or mute h.