Ciabatta - HELP!

Toast

Hi All!

 

I am a very new (and enthusiastic) bread baker. I have tried making focaccia and easier breads in the past and have been fairly happy with them. A few days ago I tried to tackle a relatively high hydration dough - a 75% hydration ciabatta following the BBA Poolish method. It was also the first time I was setting up a couche. 

Now the issue I faced was (as in the picture) - I saw some nice air pockets on the top but the bread was quite dense in the bottom. Can someone please help explain why this is happening/what am I doing wrong?