I made two today.  Both did not turn out. One was my fault  the other I just do not know what happened.  Both of the sheet cakes the middle sunk on both. 
I am just going back to using the straight mixes.  If it sinks a little how can you use fondant on it?  Won't it show the dip and you put fondant on it? Why do you think it is not turning out?

FYI The WASC recipe was developed for cake mixes that weighed 18+ ounces.

It sounds like you bought the new cake mixes that weigh 16+ ounces.  Pillsbury, Duncan Hines and Betty Crocker are ALL making this change. That alone would cause the middles to cave in.  

People are using two corrections: either weigh/measure a fraction of a third cake mix, or add one instant pudding mix per cake mix. 

What I was told is add 1/3 C cup of extra dry cake mix.  That is what I do.  Per box.

1st of all *which* WASC recipe did you use?

How much batter did you put in the pan - exactly what size pan and how much batter?

I'm very sorry to hear you have had problems. Yes, my *original* WASC recipe is based on the larger sized mixes so one must add more mix from another mix to make up for the loss.  Not only have they reduced the size, but they have changed the type of rising agent used so we are all going to have to adjust and experiement until we find what works best.

 Did you use wet baking strips or heat nail?  I'm also wondering if your oven is working properly.  It's possible something has happened there having had trouble w/more than one baked at basically the same time.

As for using a cake that has a dip in the center - you can fill in the dip w/icing before covering w/fondant.

I use the one with the oil.  I cut the sugar back to 1/2 cup.  For the bigger cakes, I use a baking nail and I make sure I don't open the oven during the first half hour.  I let it go until I can smell it baking.  If you pull it out too soon it will sink.