The chip is certainly capable of putting out both signals at the same amplituded, so the doubt is either in how you are programming it, how you set up the chip with external hardware, and how you are measuring it.
Are you using ADI software to program it on their demo board, or is it your custom software on your board?
What is the clock frequency the DDS is using? You will need at least a 150 MHz clock.
Are you using an output lowpass filter, and if so what is its cuttoff frequency. How many db of attenuation does it have at the alias of 55 MHz?
How are you measuring the output? Analog oscilloscope or digital oscilloscope? Is your sample rate high enough to avoid scope aliasing? Does your analog lowpass filter elimininate the chance that the oscilloscope is seeing an aliased frequency?