Chemical reactions occur when reactants collide. For what reasons may a collision fail to produce a chemical reaction?

One of the postulates of collision theory of reactions is
that the reacting species approach at the correct attitude. e.g for the reaction between
monobromomethane (methyl bromide) and sodium hydroxide to make methanol the hydroxide
ion has to approach the methyl bromide molecule from the side opposite the bromine atom
for best chance of reaction occurring.