No USB device detected

I got a new (to me) computer. I installed Slackware, built a new kernel on the machine. It fails to detect any USB device, which includes my only keyboards and mice. I figured this out by putting lsusb (and dmesg and lsmod) in rc.M, saving the output to a file. lsusb lists only the hubs, no attached devices. I forced loading of the relevant modules; they load, but do nothing because they see no devices. When I boot from a USB drive they work. What is failing to happen?