Field Effect and it's effect on basicity on amines
The basicity of amines has to do not only with the degree of substitution with electron-releasing alkyl groups but also factors such as steric hindrance and degree of solvation. 

The electronegative nitrogen is able to isolate electron density from the alkyl groups. So having more electron-releasing alkyl groups to stabilize the positive formal charge created by the abstraction of a proton from the protic solvent might seem like a good thing. 

On the other hand steric hindrance due to bulky alkyl groups reduces basicity. 

Also, having more alkyl groups reduces solvation due to hydrogen "bonding." Consider ammonia, $\ce{NH_3}$. This molecule can form four hydrogen bonds. On the other hand, a tertiary amine that has been protonated can form only "good" hydrogen bond; one might argue that the hydrogens on the alkyl groups can form hydrogen "bonds", but these are obviously going to be weaker "bonds" compared to the ones among ammonia molecules.