My dissertation is linked to arXiv but its citations are not found in Google Scholar!
Google Scholar, unfortunately, makes no particular guarantees about either timeliness or accuracy. I find that it is best understood as a byproduct of Google's business model, rather than a proper fully-supported tool.  

As such, it is not surprising that in my experience sometimes documents will take months, or even more than a year, in order to get properly sorted out by Google Scholar.

In the meantime, I would suggest to not worry about it too much: Google Scholar is not a definitive index, and should be used only to get a relative impression of the significance of a body of work, not to seriously evaluate bibliometrics.