[Dave Clark]

Try this:

• Select your object or objects with the transparency problem
• Click on the Transparency window options (top right corner)
• Click Show Options
• In the Transpancy window, if it has a little tick box filled with blue in it then one or more of the objects have different isolation blending or knockout group
• Click the blue tick box on and off

This fixed it for me because it removed the different isolate blending

If you want to know more I found the information here;
https://blog.infiniteskills.com/2012/08/adobe-illustrator-cs6-tutorial-opacity-and-blending/