Yes you need to explain that it has been 6 weeks or more since the GC key update said they mailed your card. And you have contacted IRCC regarding this matter who advised you to send this SD.I am in the same situation.
I am planning to fax a solemn declaration.
Did you write anything for point no 3
"Under the following circumstances (full information must be provided)"
Incidentally, under "efforts made" it's better to mention that you looked for your PR card in your mail, neighbors, post office etc. and no one received this card.