Qubee wimax net connection speed down

I am Rashedul islam from Bangladesh. I got many solutions from this site. Now I have a problem that’s why I am share with you all. I am using Qubee wimax internet connection. I used an unlimited package with speed 512kbps. But after using 15 days my internet speed slow down. I was happy with the first time speed. But now I become so upset.
What is the main problem?
.
Anyone
suggest me please
