Raquel, other factors can also be part of the issue including different forms of learning disorders. The more you get to know your students as individuals the more you can adequately assess their need for more assistance. For some students they simply do not know how to study or know how to prepare for major assignments including assessments while for others they prepare hard and know the materials but the methodology of assessing their knowledge may not conform to their talents. I have mentioned this in other posts and it seems very appropriate here - One size does not fit all and the more you know your students the better you can guide and coach them towards the level of success they want. James Jackson