I have had that same problem with several A2000's- the thumb loop "bottoms out", or just doesn't have enough adjustment to get it tight around my thumb.
On the one glove I used as a gamer, I had to do a little surgery. Opened up the glove and used a razor knife to cut about a half inch piece out where the ties meet the loop, essentially making the ties 1/2" longer and giving me enough adjustment.
Something like this....