In this case it is one reason but we don't know the reason. You can say some reasons too but in this case you usually know but don't want to tell.
For some reason, she isn't talking to me. I don't know why but since she usually talks to me and I don't usually upset her there must be one big reason but I don't know it.
I won't tell you all the reasons I have but I can't join club any more.
Thank you!!
There are several reasons or some reasons that we failed. Here they are: ....
Some reasons behind the plan include ... , ..., and ....
For some reason, I can't get the car to start.
Let me try. When did you last check the starter?