Why don't the echoes remain forever?

I have been trying to find out the answer of this question. The case is, I and my friend shifted to our new 2BHK flat 6 months before. Actually, when we went to see the flat first time, at that time when we were talking to each other, we could listen the echoes of our sentences. Also, the people who came to see our new flat, while talking to each other the echoes of the voice could be heard at least 3 to 4 times regardless of the state of the doors and windows (Open or Closed). Now, It has been 8 months since we came here. We no longer hear the echoes now a days. What is the reason behind this? Why don’t they remain forever?.

Anyone know the scientific reason behind sudden last of the echoes?