Are you running the laser diode at ~2.5V (or whatever it says on the spec sheet for that diode)?
Can you try hooking up an ammeter in series with the diode to see how much current it draws? Are you sure it is supposed to be red and not infrared (which is far less visible)?
If it draws a fair bit of current (say more than 100mA) and is still that weak, the diode is damaged and needs to be replaced