I would love to help out on this one, but without a bit more information it is difficult. The manufacturing process for all Hen long stuff is so shoddy it could be a million and one things.
If the tank was fully functional when you bought it, the first things to check are the obvious. (If it is under warranty dont bother trying to find the fault -just send it back and ask to be refunded then save for a Real Tamiya)
1. Batteries fully charged (both tx and rx)
2. Check the connection for the arials on tank and transmitter