Why are USB ports sometimes not interchangeable?

that is part of the standard usb. its dues to various devices inputs. just because you put a device in one usb, doesn't mean it will be a standard slot for it. remember usb ports is PnP(plug and play). make sure some of your device is 1.1 and 2.0 backward. some usb2.0 device won't work on usb1.1 ports. please check your system specs and also check the manufacturer of each device.