Why don't most of the gases in the earth's atmosphere Contribute to the greenhouse effect

 Before explaining this lets understand what happens in green house effect.

Earth is constantly bombarded with enormous amounts of radiation, primarily from the sun. These radiations come on to our Earth in different forms  mostly like visible light, ultraviolet (UV), infrared (IR) that are invisible to the human eye.

 It is said that about 30 % of the radiation striking Earth's atmosphere is immediately reflected back out to space by clouds, ice, snow, sand and other reflective surfaces.

The remaining 70 % of incoming solar radiation is absorbed by the oceans, the land and the atmosphere and this process  make them heat up.

As they heat up, the oceans, land and atmosphere release heat in the form of IR thermal radiation, which passes out of the atmosphere and into space.

There is an equilibrium existing between the incoming and outgoing radiation and this equilibrium helps to have an average temperature which is suitable for the existing of  living beings.

Certain gases in the atmosphere have the property of absorbing infrared radiation. The infrared radiation strikes a molecule such as carbon dioxide and causes the bonds to bend and vibrate - this is called the absorption of IR energy. The molecule gains kinetic energy by this absorption of IR radiation. This extra kinetic energy may then be transmitted to other molecules  and causes a general heating of the atmosphere. this is what happens at green house effect.

In order for molecular vibrations(molecular vibrations mainly of two ways as stretching and bending) to absorb IR energy, the vibrational motions must change the dipole moment of the molecule. All molecules with three or more atoms meet this criterion and are IR absorbents.

eg;CH4,