After some discussion with [Robert Smith](https://vi.stackexchange.com/users/210/robert-smith), here is the conclusion:

The problem was not the mapping nor the command per se. The problem was the timeout that the vim configuration was using.

The timeout is configured by the options: 

    timeoutlen   : mapping delay
    ttimeoutlen  : key code delay 

For example, in MacVim 7.4 the default configuration is:

    timeoutlen=1000
    ttimeoutlen=-1

It means that for both mappings and key codes, the time of 1000 ms is applied. Vim will wait 1 second before the mapping/key code completes.

The set of values that solves the problem are:

    timeoutlen=1000 
    ttimeoutlen=10

It means that a mapping will wait 1 second to complete (as before) whereas a keycode will complete after only 10ms.

When checking for a non-firing mapping, it should be good advice to see those values.