FFT CMSIS always showing zero for non-multiples

Running 4096 point FFT with 8 cycles of data. as per my sampling rate the frequency spread is 6.25Hz. So Expecting the fund at 8th bin which is coming properly. But expecting 175Hz spectrum at 28th Bin. But 28th bin is always zero. Instead some value is showing at 24th bin and some at 32nd bin. 

Then I tried with 160, 180Hz. But other than 8, 16, 24, 32... all other bins are always zero.

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