You have to give a good reason why you're not able to attend the test invite. Only they can deem it a sufficient enough reason. Otherwise, you run the risk of having the application rejected. So just call them and ask them.
Might be many reasons, like where you are, what your application contains, how easily they can verify all the information, etc. It could be any reason, really.
It could be anything! There are people who applied in December and still haven't received their processing status yet. If you don't hear anything after 15 months of it being sent out, then make the call to ask what's going on. It could be as simple as they just haven't gotten to it yet, to as complex as they can't verify certain information or the verification is taking a long time to come back. It's frustrating, I know; I've lived here 11 years as a permanent resident, married a Canadian citizen, have a Canadian education, literally Canadian everything except for citizenship, but I STILL have to go through this whole process too.
