I have been stuck in chapter 28 stage 3(row G) of the normal campaign for months. I have 3 starred this stage and the previous ones but it still will not let me proceed. Screenshot attached for clarity.
Click at the small dot between numbered stages.
There is one under that middle star, and other one is clearly visible.
And if stuck because enemies are tough…
It shouldn’t be technically a problem 
But you can post screenshots with your heroes.
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Click on the last stage you completed and hit the next stage button. It helps select the stages covered by text.