When my compressor stopped working, it was because the o-rings were leaking. It appears if even a tiny amount of refrigerant escapes, the A/C clutch won't engage. I don't know if this is your problem, though. When you say you turned it up to 3/4, what does that mean? Do you mean you turned your manual temp knob to 3/4 cool, or you increased your fan speed? I wouldn't imagine fan speed has anything to do with the compressor turning on/off, but I'm no mechanic. In any case, if this is persistent, you will need to take your car to the dealer, and you may want to mention the o-rings. It took my dealer 3 days just to figure that out, then they had to order parts.