I have the same code, running the same highcharts-convert.js and phantomjs on two servers. One produces perfect charts images the other is missing all labels. Does anyone know why or where to start looking?
This is most likely missing font packages on the failing host. Highcharts-convert uses the fonts available to it, but will silently skip labels if there aren't any available. I had this happen and running
sudo yum install dejavu-fonts-common dejavu-sans-fonts dejavu-serif-fonts libXfont xorg-x11-fonts-Type1
fixed it. I don't yet know what subset of those packages would have been sufficient, but I suspect "libXfont xorg-x11-fonts-Type1" would do it.