The famous Mufassir/interpreter of the Qur’an Abu Abdillah al-Qurtubee 671 AH said,
“If it is asked, ‘How is it that we see many evils and acts of disobedience [occurring] in Ramadan? How are these things happening if the shayaateen are chained up?’
Then the answer is that [evils and disobedience] are only lessened for those who observe the fast while preserving its conditions and etiquette. Or [one can say] that only some shayaateen are chained and those who are chained are the major ones [not minor]…and this has been mentioned in some chains of narrations. Or it can also mean that the occurrence of disobedience is less in Ramadan compared to the other months.
Even if all of the shayaateen were chained, then this does not mean evil and disobedience would not occur. There are other reasons that influences disobedience to occur, for example: people having evil souls, bad habits, and also the human shayaateen.”
Source: Fathul Baari