Missing answer boxes, done activities and grades.

Student´s grades disappear in the excel file. In the workbook, on top where the average is placed, actitivites that were already done suddenly dissappear and state that they have done less exercises than what they actually have.  Also, after several students have  finished their exercise but wanted to repeat it, although there is no limit for repetition, the answer boxes disappear and there is no way of bringing them back. Student`s can`t type nor do anything.