Hi Dave, the problem is two fold
1.All fruit contains acid, the acid in the raspberries breaks down the gelatine that sets the Jelly.
2. by adding fruit you increase the volume to more than 1/2 pint.
You need to do 1 of two things and a few more cubes of jelly or add less water.
Also shop around and look for jelly crystles in stead of cubes it tends to be a better product.
Just a bit of trivia, geletine is generaly made from pigs troters and savoury jelly (like inside pork pies) is called aspic.
Hope that helps
Chris