Does Slave Select have to be sychronous with Clock?
No, in fact there's usually a minimum time that the CS signal must be pulled high/low before you can start clocking in data.
From your datasheet (pg 19):
:
$$ t_{SPILEAD} $$ is the minimum time, in your chips case 4µs (datasheet pg 18). Make sure your transmission follows those timing requirements/transmission protocol.