WordNet sense 1 (of a quantity not able to fulfill a need or requirement):

deficient, insufficient

WordNet sense 2 (lacking self-confidence):

diffident, unsure

WordNet sense 3 (lacking or marked by a lack of confidence):

unconfident

WordNet sense 4 (showing fear and lack of confidence):

timid

WordNet sense 5 (marked by keen caution and watchful prudence):

wary