Why did Sidewinder missiles score so well before Vietnam?

Question

In the Wikipedia article about the Second Taiwan Strait crisis that the USA had provided Taiwan air forces with Sidewinder missiles. During clashes with PLA's air force, Sabre equipped with the missiles seem to have scored pretty well.

However, in next conflicts such as the 1971 Indian war, during which Pakistani Air Force used Sabre and Mig-19 jets armed with Sidewinder missiles, it seems not to have performed that well.

Even more, during engagements in Vietnam, the US Air Force and US Navy fighters such as F-4 Phantoms needed many missiles to score one hit. And a gun was asked to reinforce the capability of the F-4 at close range.

So how can we explain the differences of efficiency? I can think of some possibilities:

• Process of evading manoeuvres that allowed Indian and Vietnamese pilots to escape, while PLA's pilots did not have (because they were the first to encounter the missile)?
• Some countermeasures systems? However I did not find such systems on Vietnamese aircrafts.

Answer 1

Question:
Why did Sidewinder missiles score so well before Vietnam? Sidewinder did well in 1958 in Second Taiwan Strait crisis; but in 1971 Indian war, during which Pakistani Air Force used Sabre and Mig-19 jets armed with Sidewinder missiles, it seems not to have performed that well.
...
Even more, during engagements in Vietnam, the US Air Force and US Navy fighters such as F-4 Phantoms needed many missiles to score one hit. And a gun was asked to reinforce the capability of the F-4 at close range.

Couple of things going on in your question.

The AIM-9 Sidewinder missile which is still in use today, was developed in the late 1940's and was deployed first in 1956 by the United States Navy as a heat seeking subsonic missile. Its reputation is that of an effective reliable air to air missile, widely used and copied by countries like the former Soviet Union ( K-13 ).

Alternatively, while the early sidewinder was being developed the US Air Force developed a different missile the AIM-4 Falcon. It is the Falcon which had troubles, famously in Vietnam. It was Falcon AIM-4 which an early radar guided missile which was ineffective during the early Vietnam War leading the air-force to replace it with the Navy's sidewinder.

Over the years the hardware (the sensors, the engine, warhead) and Software has changed continuously. At times it has been more effective than other times; which has lead to studies and modernization efforts. It's early subsonic heat seeking version bares little resemblance to the modern supersonic radar guide version which remains highly regarded. It's effectiveness or ineffectiveness likely has a lot to do with which version is used vs which era's planes.

Two influential studies on the sidewinder where the USAF Red Barron's Report
and the USN's Ault Report both of which helped guide the evolution of this weapon..