Hi Bookworm, I'm a total amature at painting and the net, so I can only give you advice from my own experience! I did have the same problem as you with a new bottle from SAA Home Shop.
Thinking it was my in-experience and being a bloke, I then decided to read the instructions, there isn't any! So here is how I have solved the problem: (1) shake bottle really well for a good minute or two before using. (2) when leaving for a while, store upside down in your art box/draw and out of direct sunlight.
I have now had my bottle for approx 18 months and have not had any more problems.
I'm not saying this will work for everyone, but it worked for me.