We present a case study using an online CALL game with secondary school students, where we examine the question of how different factors, such as gamification elements, difficulty of exercises and personal background can influence students' behaviour with regard to performance and motivation. For this study we used CALL-SLT, a speech-enabled online CALL tool for beginner learners of English. The content consisted of 8 interactive and dialogue-based lessons that were based on an English textbook commonly used in German-speaking Switzerland. The lessons are designed to help students practise their productive and receptive skills in a non-judgmental environment, where simple feedback (correct vs. incorrect) is provided in real-time by the computer. The tool was used in four secondary school classes (approx. 50 active users) all across German-speaking Switzerland for a duration of four weeks each. The experiment setup also required the students to take a placement test at the beginning and at the end of the experiment phase and to fill in a questionnaire on personal background at the beginning, as well as a qualitative feedback questionnaire at the end of the experiment. We examined three specific aspects with regard to their influence on motivation: (1) the influence of gamification elements, such as scores and performance-linked badges, (2) the influence of the exercises' difficulty and (3) aspects of students' personal background.