This paper reports on an empirical study that investigates second language (L2) acquisition of English single wh-questions by adolescent Japanese-speaking learners by focusing on how the features involved affect these learners. A picture stimulus production task was administered to first language (L1) Japanese junior high school and university students (aged 12-20) at beginner (n=42), post-beginner (n=60), and intermediate (n=30) levels of English. The results reveal a clear asymmetry in accuracy between subject wh-questions (7.3%) and object wh-questions (74.5%). Additionally, two kinds of redundant lexical items (be/do and a non-wh pronoun) were noted, but only in subject wh-questions. The findings suggest that Japanese L2 learners across these proficiency levels have difficulty noticing the differences in feature reassembly between the two types of wh-questions. This is in line with the Feature Reassembly Hypothesis (FRH), which can account for the asymmetry observed in the acquisition of English single wh-questions by Japanese young instructed learners. It is proposed that complex L1-L2 differences in feature reassembly can be a source of persistent learnability problems for L2 learners.