I also applied for Canada Immigration from Mexico City. I was using a representative/agent for my processing.
I had applied for extension similar to your case 3 times and I was given the same everytime.
PMM has already given the process to do that.
However, I wanted to share my experience with Mexico City consulate.
The immigration officers are pretty slow in responding to the queries.
For once, I got the extension letter from them on the last day of my given time period.
So you have to be extra patient. Even if you send them inquiry mails, they will reply after couple of weeks with a template response to "Be patient and they will get back soon"
The other thing is that give them proper reasons and supporting documents if required.
For me, my agent asked for a original request letter with my signature. She told me that is needed when we have to ask for extensions.
Do let us know how your extension request is attended to.
Good Luck.
Regards,
Sharat Joshi