Prospective memory research often assumes that unfulfilled intentions reflect memory failures, but the reasons behind self-reported daily intention non-completion remain underexplored. In this study, we investigated the relative contribution of forgetting versus other factors to unfulfilled daily intentions and examined how intention-related characteristics predict the type of non-fulfillment. A total of 102 distance-learning students completed smartphone-based intention diaries across two five-day periods, reporting on 1823 non-routine next-day intentions and providing ratings of importance, perceived difficulty, memory confidence, and reminder use. Mixed-effects multinomial logistic regression revealed that memory confidence and perceived difficulty reliably distinguished non-forgetting reasons, whereas perceived importance had no effect. Reminder use was selectively associated with external causes rather than forgetting. Cross-validation with a publicly available dataset (Scott & Gilbert, 2024; N = 409 intentions) largely replicated these patterns. Overall, within the present sampling frame, forgetting accounted for only a minority of self-reported non-fulfillments (~ 10-14%), suggesting that situational and motivational factors, rather than memory failure, are primary drivers of non-completion in daily life. These findings challenge the assumption that unfulfilled intentions in naturalistic settings are predominantly due to forgetting and highlight the importance of considering multiple psychological and contextual aspects in prospective memory research.