The Morale is a game mechanic that decreases over time, regardless if you played matches or not and of their result in some cases. For that reason it is normal to see one number in the preparation level and after a certain period of time to see completely different. The only way to be restored to the fullest, is by using boosters. Keep in mind that your players may also lose morale because of some bad tackles, yellow/red cards, missed shots, etc. This is why even if you win your matches, your players may not feel at their best.