I just checked 2 python plugins I developed that both worked fine in R19, but in R20 and R21, one of them doesn't show up. I copied the 2 plugins, without changing anything, from the R19 plugin location to the plugin folders in both R20 and R21. I checked the python console and there were no errors displayed. One of the plugins just goes unnoticed by C4D in both R20 and R21.
Any ideas why?