LMX2491: some question about the Denominator configuration about LMX2491

Part Number: LMX2491

hi all;

when i use the LMX2491, I have some questions about the Fractional Numerator and Denominator Calculation.

PD frequency = 50 MHz

R = 1 

N = 31.25

output frequency = 1562.5 MHz

So i config the R divider = 1, N divider = 31,  the Numerator = 4194304, the Denominator = 2^24（the register 0x16~0x18  i write all zero) , the fractional order = 4 ( the N divider is 31.25)

But the PLL always lock at 1550MHz or 1600MHz , just like the fractional mode is lose effectiveness

If i only change the Numerator  to 1 and the denominator to 4, the PLL can lock at 1562.5MHz

So I calculate the fractional numerator often use  the decimal fraction multiply 2^24(16777216)。 Does this method has any error?

Thank you 

look forward for your help .