In this paper, we study the link between di culty and player's motivation in two games developed by Ubiso ®: Rayman®Legends and Tom Clancy's e Division®. We describe a method to estimate players' di culty over time and link it's time varying e ect with players retention. Results con rm ow and self-e cacy theory. Also, for the rst hours of playtime, results di er between the two games. We explain that discrepancy with regard to a ribution theory : in Rayman Legends, failure can be mainly a ributed to the player skills, while in Tom Clancy's e Division, avatar's strength plays a fundamental role and can always be relatively quickly improved.