Jorge: I have an iMac and a digital concepts 9in1 reader. Plugged into the computer itself (right side back). MOST of the time when it has problems it is because the USB cord has worked slightly loose at the card reader end. Occasionally the card needs to be removed and reinserted.
As mentioned, card readers need to draw power through the USB cable. So they also will not work via a "hub" unless the hub has its own AC-block power supply. Unless of course the card reader has an AC power supply itself (most don't).
With my reader, the process is:
Insert card. An LED light should come on on the reader, and then start to flicker as the data connection is made with the computer. If no light comes on, that's the sign that the cord is loose (no power). If the light comes on but doesn't begin to flicker, that's the sign the power is OK but the card is not making correct contact.
If the contact is good, an "external floppy drive" icon shows up on my desktop in the upper right (Or as close to it as the other icons will allow) titled "NoName" (at least with SD cards from the M8). Inside that are the DCIM and 100LEICA folders for the pictures.
My home 9in1 reader requires that SD cards be inserted contacts-up, face-down. The 2in1 MS/SD reader I have on my PC at work is the opposite.