Why +M effect of -NH2 is more than that of -NHR and -NR2? Why does $\ce{-NH_2}$ shows more +M effect that $\ce{-NHR}$ and $\ce{-NR_2}$? My first conclusion was totally opposite of the reality. I thought, as the alkyl groups are attached to the Nitrogen the lone lone pair on nitrogen should be easily available. But it doesn't seem to be happening. What is the actual cause?
It is because the bulky alkyl groups provide repulsion to the lone pair such that their orientations change. Due to the change in orientation the %s character changes and the lone pair becomes less available. This is not the case with hydrogen.