DC 9.21 vs DC 9.50...

After implementation of DHCP, after perticular data processing I am calling line to be down. When I again want to do something online, I called DHCP routine again to configure network setting. and follow same procedure ie data processing and again calling line down.
In case of DC 9.50, all code function well… but if I use DC 9.21, it will not configure DHCP second time and hangs after sock_init()…
what would be the reason. Success rate with DC 9.50 is 100%. but DC9.21 fails.