About the output of AD9951

Dear Forum

   I refer the official manual to design AD9551 ,and my output filter is refer AN-837 as follow,

 now my problem is when my output frequency is 100KHZ,it output normally,but when my output frequency below 30KHZ,it  hadn't output,the signal

at primary is 1.75V(DC),and the signal at secondary is zero,my balun is ADTT1-1+,I want to know why?