I keep getting under-prooved dough when I use rapid-rise yeast?
I recently made _Classic German Baking's_ Chocolate swirled guglehupf. The recipe calls for instant yeast and has you mix it right into the dry ingredients. Honestly I had trouble right from the start, after 10-15 mins of kneading my dough was still mottled (and never truly became smooth and elastic), left it to rise for a little over an hour as the recipe called for but didn't get much rise in the first or second prove. The end result was a dense guglehupf with slightly underbaked corners (in spite of an hour long baking time - also called for in the recipe).
Is there some tip to rapid-rise/instant yeast that I may benefit from?