What should we do if our students cannot reveal their ideas in to a model

Model is a tool used for mathematician to reveal their ideas and thinking, and also to help them as tools to solve questions or puzzle. In Realistic Mathematics Education (RME), we know  well about the using of context and model. There are two different used of model in RME, model of situation and model for students’ thinking. In model of situation, students try to describe what they find in the context given, just exactly similar with the problem found. Usually, students tend to draw the situation in their paper. In the other hand, model for the picture are becoming more abstract. It is followed by their understanding more toward the problem. Students make a drawing not exactly the real situation but more close to the abstract calculation. I would like to give you an example; Corry just celebrated her 8th birthday. She plans to give  5 heart stickers to her best friends, for which each 5 stickers will be put on an envelope. Now, she is struggling to know,  how many heart stickers that she should buy if she had 20 heart stickers.  For more clear information, I would like to invite you to this following picture: