Just a thought, Conne:
I had great trouble opening a bottle of bleach some time ago. I squeezed the two sides in and tried to turn the top but it wouldn't budge. I squeezed even harder, tried again and still got the same result. After several minutes I gave up and opened a different bottle of bleach instead. When that one was empty I tried again with the first bottle and still couldn't open it. Eventually, though, I discovered that I'd been squeezing the top TOO hard all along. By giving it just the GENTLEST of squeezes I got the top to turn easily!
So perhaps you're experiencing a similar problem? (i.e. instead of looking for something to squeeze it harder, try being gentler with it instead!).