That question poses Two other logical questions.
1) Are you using a 4AA battery cell unit or a 6VDC Rx pack for the receiver ?
If you use The 4AA battery cell unit, then most likely your batteries are not giving out enough power and the failsafe is doing it's job properly.
Information
A failsafe kicks in at around 4 volts so if it keeps kicking in regularly, try running a 6 VDC (Thats 6volts Direct Current) Rx Pak.
2) is your vehicle within "Range".
If your vehicle is fairly close, say 30 feet away, and the failsafe is kicking in, then it is your battery choice that is causing the problem, either the 4AA's are not giving out enough power, or the 6 VDC Rx Pak is flat and needs charging, with the failsafe kicking in and doing it's job. If, however, your failsafe kicks in at a fair distance, then it may be that your vehicle is "Out of range" and the failsafe is doing it's job as it should be.