[Carry_Okie]

If the plane is low enough, it might be that the missile doesn’t have the time to lock and correct course.

[Hulka]

Low you can avoid being seen until you pass overhead or never are seen. . .in a wooded environment. In the desert. . .not so much.

Getting locked on the missile flies a pursuit curve, chasing the heat signature. Nothing to do with time to track/adjust. . .it comes off the rails already tracking.