Hello,
My initial test date was February 8, 2018. I received the test date invite via email.
I couldn't make it. I replied the email I received requesting a new test date and gave them a time frame (March 12-16, 2018)
I received no reply.
I called IRCC who suggested to submit a web form as well as send regular mail to my office (Scarborough) which I did.
On February 7, 2018 (one day before the test), I received an email stating that my request was received and they will try to accomodate me in the selected time frame.
On February 21, 2018, I received the new test date invite via email. Scheduled on March 13, 2018 (within my requested time frame)
I recommend doing the same.
Feel free to ask any questions.