Word strength "decays" after it's been a long time since we last were exposed to them. To keep the word strength in tip-top shape, we recommend practicing and "strengthening" the skills with practice. So the gold tree does not stay gold forever. ;]
It is probably because they didn't do so well, not that I am saying your students are bad, because if they got many questions wrong then they will need to strengthen them to be able to remember them better.