Many studies have explored the second language (L2) acquisition of relative clauses (RCs) and whether L2 speakers transfer a resumptive strategy from first language (L1) to L2. While evidence seems to suggest that there are significant L1-L2 differences in the processing of RCs, relatively little is known about the source of non-target-like L2 behaviour. The present study investigates the grammatical acceptability of different RC types in L2 English and whether reliance on a resumptive strategy is a syntactic or processing issue. The participants included 71 L1-Persian L2-English, 52 L1-French L2-English, and 44 native English speakers, who completed a proficiency c-test, a grammaticality judgment task, and a reading span working memory (WM) task. Unlike French, which is similar to English in the syntactic derivation of RCs, Persian is a structurally wh-in-situ language that syntactically allows resumption in direct object and object-of-preposition RCs. The results showed that unlike L1-French speakers, L1-Persian speakers were more likely to accept resumptive pronouns in L2-English RCs; however, both L1 and L2 groups overwhelmingly preferred a gap over a resumptive strategy. The results suggest that given sufficiently high proficiency and long immersion experience, L2 speakers can match native speakers in terms of RC syntactic representations, suggesting that the issue faced by learners is a processing issue rather a representational one as suggested by the Interpretability Hypothesis.