Several studies have reported that cognitive training can lead to improvements of complex mental skills such as intelligence. However, attempts to replicate these findings have not been very successful with many studies reporting lack of transferable effects on cognitive processes unrelated to the training task. On the other hand, transfer effects on cognitive processes closely related to the training task have been more commonly reported. In this study, we investigated the effects of a frequently used working-memory training programme on fluid intelligence and specific executive functions (updating, inhibition, switching, the focus of attention, and sustained attention). We remedied common issues with previous training studies by using an active control group, using more than one instrument to assess each function, and including a larger sample size. The experimental group showed significant improvement in the training task, indicating strong practice effects. However, no evidence of training-specific transfer was found in any of the variables investigated, and we could not find any of the previous improvements reported. Participants in both the training and control group showed post-training improvements in most of the outcome variables, suggesting that practice effects can be found even when a task is only performed twice. We conclude by discussing possible explanations for the differences between our results and those reported in prior studies, and recommend that any claims of improvement should be supported by studies capable of replicating them.