A Featured Transition System (FTS) is a formal behavioural model for software product lines, which represents the behaviour of all the products of an SPL in a single compact structure by associating transitions with features that condition their existence in products. In general, an FTS may contain featured transitions that are unreachable in any product (so called dead transitions) or, on the contrary, mandatorily present in all products for which their source state is reachable (so called false optional transitions), as well as states from which only for certain products progress is possible (so called hidden deadlocks). In this paper, we provide algorithms to analyse an FTS for such ambiguities and to transform an ambiguous FTS into an unambiguous FTS. The scope of our approach is twofold. First and foremost, an ambiguous model is typically undesired as it gives an unclear idea of the SPL. Second, an unambiguous FTS paves the way for efficient family-based model checking. We apply our approach to illustrative examples from the literature.