It was proven that the Prophet (peace and blessings of Allaah be upon him) said: "When Ramadaan begins, the gates of Paradise are opened, and the gates of Hell are closed, and the devils are chained up." Narrated by al-Bukhaari, 3277; Muslim, 1079. According to al-Nasaa'i: "The most evil jinn are chained up."
This does not mean that the devils have no influence at all, rather it indicates that they become weaker in Ramadaan and are not able to do what they are able to do at times other than Ramadaan.
It may be interpreted as meaning that the ones who are chained up are the most evil devils, not all of them.
Al-Qurtubi said: If it is said, how come we see many evil actions and sins being committed in Ramadaan, for if the devils were chained up, that would not happen?
The answer is that (evil actions) are reduced on the part of those who fast meeting all the conditions of fasting and paying attention to the etiquette of fasting.
Or some of the devils - namely the most evil ones - are chained up, not all of them, as previously mentioned in some reports.
Or it may mean that evil actions are reduced in Ramadaan, which is something that we can see. Even though evil actions happen in Ramadaan, they are less than at other times. Even if all of the devils were to be chained up, that does not mean that no evil action would happen, because there are other causes of evil action besides the devils, such as evil souls, bad habits and the human devils.
From Fath al-Baari.