All of the above :yeahthat:I use cotton twine. Cut the excess band off right next to the knot. Never had any slips in my life ever. Constrictor knots are quick and efficient. For this reason I have never switched to anything else.
Plus, I've found that cotton twine, opposed to rubbers strips (or other tying medium made out of rubber) makes bands last 3 times more.
From what I've seen in the pictures, your folding technique seems to be the problem. Perhaps you're not folding the bands properly when tying.
Best regards ...Q