After some discussion with 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.

After some discussion with 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.

After some discussion with 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 the mapping and the key code, the time of 1000 ms is applied. Vim will wait 1 second before the mapping/key code complete.

The 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.

After some discussion with 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.