EFL learners commonly infer word meanings while reading. However, previous research suggested that a large number of lexical inferencing attempts were far from success (Nylander, 2014; Qian, 2005). This study aims to examine possible factors of failure in lexical inferencing, with strategy use as a main focus. Eight participants were asked to read an authentic academic text and infer meanings of unknown words. A retrospective interview was conducted to investigate the words inferred, meanings obtained, and lexical inferencing strategies used. This study confirmed the high amount of failure or less successful inferencing attempts. Qualitative analysis of the inferred words in context and strategy use in the less successful cases suggested that lexical inferencing is a complex and demanding process. It intertwined a combination of underlying factors including strategy use, characteristics of the inferred words, contextual clues, and such learners' factors as vocabulary breadth and depth, grammatical knowledge, and background knowledge. These findings imply a pressing need in enhancing effectiveness of lexical inferencing of EFL learners.