Indexed colors stop at 16

I’m rendering in 8 bit color index mode, and for some reason colors don’t show up after index 16. I imagine palettes aren’t set or something, but I don’t care about that since I’m glReadPixel-ing the data and dealing with the palettes manually. I just need to set a number of indices or something. I’ve tried the index mask thing, but it was set to 255 and setting it to 255 again didn’t help. Any suggestions?