# 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?