AD9213 Eval - 12-bit operation

We have been using the AD9213 with the ADS-V8 FPGA card. Until recently I had been running with the DDC off, full BW mode with no issues. I have begun to characterize the part with the DDC in decimate by 2 mode, 2 virtual converters to convert to complex. With N' = 16 things program fine on the ADC and the FFT analysis tool works great. With N' = 8, same thing, works great. With N'=12, no matter what the sample rate is, etc, the ADC appears to accept the settings but when I go to the Analysis tab and attempt to capture, the analysis tool sits there attempting to capture until it hangs up. I can't ever get a capture with N' = 12. Is there a solution to this or any suggestions on something else I could try?

Thanks!

Josh