I am having this problem and googled it - another person had also posted about no longer being able to crack their back and here were the 2 responses:
Keep in mind- when you "crack" your back, while it feels good ( endorphin release), it is not actually properly aligning your back. As a result, after a while, your back will actually get "stuck" in a position that you can not "twist" out of. In order to put your back into proper alignment, you really need to see a chiropractor. They specialize in properly aligning your back. They will be able to help you get rid of that middle and upper back pain.
This happened to me when I was pregnant. Being unable to twist around meant that I couldn't crack my back anymore, and then after the baby was born, it was like the joints had fused up. I had to see a chiropractor to get the flexibility back.
I thought I couldn't crack my back anymore due to being over weight, the last time it cracked well was actually with a chiro and a I was pregnant at the time. It was better than sex - no lie! It cracked everything in my body from below my tailbone - up the spine and to my neck. I couldn't get off the table -- and told the guy he should charge way more than they did!!!