What could cause a relay not to release immediately?
Relays never open ‘immediately’ there is always a delay, usually hundreds of microseconds to several milliseconds, depending on the relay construction and how the stored energy is dealt with.
Possible reasons for a relay not opening include problems in the driver circuit (coil current not falling to essentially zero), contacts sticking (typically a result of damage due to excessive inrush current or a momentary short) and debris within the relay (very unusual).
For example, omitting the flyback diode can result in exceeding the safe operating area of the driver, causing cumulative damage and eventual increase in ‘off’ current to the point where the relay may not drop out reliably.
Sticking contacts can manifest as the relay remaining closed until it is tapped mechanically.