Large irregular holes only on the ends, not the center??

Hi,

I recently made Tartine's Basic Country Bread (for the 3rd time) and the loaf was dense in the center but on the edges their were the large irregular holes ............what I am looking for.

Why was it so dense in the center?  

Why was the large irregular holes only on the ends?

Any suggestions what I did wrong?

Thank you for your time!