I have built a plot in which I draw lots of scattered points (QCPCurve with line style = lsNone & scatterStyle = ssDisc) and many of these points were not shown. I do not know if it is an expected behavior. In any case, I succeeded to show all my points by calling setScatterSkip(0) even if it should be the default value according to documentation.

I hope this can help if it is not an expected behavior.