I found with mine, if I do the top up too tight, they leak! If I don't over tighten, they don't leak
Thats really interesting, I always do mine up tight as I thought it would stop them leaking, will have to try your way.
I found the glass bottles really bad for leaking.
My boys have a Ferplast Sippy bottle, they arent cheap but well worth it no leaks whatsoever!