Public Variable Reference

This is an answer to the answerhub question here:

"The issue is probably that you didn't set the reference to your blueprint in the details panel of your BP-B. You could have many different copies of BP-A, so it doesn't automatically pick one for you.

Make sure that the variable is already public, find it in the details panel after selecting BP-B in the world outliner, and set it to a reference of BP-A ."