This study is an attempt to investigate the issue of failure in second language acquisition syntax. The Failed Functional Feature Hypothesis (FFFH) proposed by Hawkins and Chan (1997) claims that parametric options not instantiated in Li feature system of functional categories would lead to persistent difficulty even after a long time of exposure to L2 input and at very advanced levels of proficiency. This paper has focused on the acquisition of English interrogative clauses by adult Persian speakers. Based on the analysis of the elicited data, I argue that failure occurs where an L2 structure (trigger) is compatible with both Li and L2 representations; however, where such compatibility does not exist L2 learners have a chance to acquire the new option.