I use these seals on products sold in my shop. They work well but once in a while we would have one that wouldn't stay sealed. We have found that slightly warming them (we use the heat gun that is used to do the shrink wrap) before sealing down has worked perfectly, every time.
These don't seal when I tighten the cap no matter how hard I tighten them. I have to press down on the seal with each bottle to get them to seal at all. Very cumbersome. Look elsewhere.