Luchinss (1942) classic Einstellung (mental set) phenomenon has been demonstrated across a wide variety of samples and problem solving tasks. However, it is unclear how increased practice with the initial set affects subsequent performance. Although Luchins anecdotally reported no effect of increased practice with set problems, current theories would suggest otherwise. In this study, we varied the number of set problems (0, 1, 5, 10, 20, 40) in the water jar task and examined the effects on subsequent performance. Results suggest that problem-solving flexibility decreases linearly as the number of set problems increases. Contrary to the predictions of a dual-process theory, we found no evidence of a U-shaped association between flexibility and number of set problems, at least not in the range of 0-40 problems. These findings highlight the need for further investigation into the situations in which practice and automatization lead to change resistance versus reflection and conscious control.