USB Problem With Printing, etc.?

I use my 4 usb ports for the usual things; printer, pc card, etc. Why is it that I must use certain ports for certain devices? I use 2 different printers, however, only 1 port works for printing. Likewise only one port recognizes my pc card (its not the same one which recognizes my printers). I have unused usb ports, but they won't run either of the printers. Please help!