@ The missiles were an old Soviet design and not as smart as newer models. They could track an aircraft only if within 10 degrees of its axis of flight (from the rear). Their electronics might have been corrupted or other parts could have been corroded.
@ Israeli airliners have unspecified defenses, probably infrared jammers.
@ Modern turbofan engines used on airliners (to reduce noise) surround the hot exhaust with a wall of normal air, reducing the cone from which a missile can track them even more. The mixing of the exhaust with this bypass air also cools it enough to reduce the chances of lock-on. As the pilots saw smoke trails, the missiles presumably worked but simply did not lock onto the aircraft. Older aircraft get the same defensive benefits with the addition of "hush kits".)--Stephen V Cole
Why did the missiles fired at an Israeli airliner in Kenya miss? We will never know, but there are several possible factors: