I have been stuck at 80.0 for about 5 hours with no gain. It is set to go up, not skill capped, wearing 27 armor... Was wearing 20 armor for 3 hours, no gain. Dropped it down to 13 armor for an hour, no gain.
I know this might seem silly but it happened to me so many times I have to ask, did you make sure you have room to gain skills? If at 700 real, make sure you have some unwanted skill pointed down.
I don't know how accurate our system is to OSI Renaissance stealth mechanics, but I believe that 13 armor may still be too high. If you are failing 100% of the time, you will not gain. I tested skill-gain with this skill during the Alpha/Beta & was able to reach 100, though I don't remember the specific armor value I was using.