SSI Slave Receives only 0x00

Hello,

I have a board with 2 processors:

1. F28M3552C1 (Concerto) as SSI Master.

2. LM4F232 as SSI Slave.

8 bit mode 400000 bitrate.

I am sending bytes from the master to the slave, and also on each slave receive it sends back a byte.

in slave RXFF interrupt I receive the data with SSIDataGet,

But all I get is always 0x00, What can be the problem ?

When I send a byte back from the slave to master, the master get this byte successfully.

I look at the data in oscilloscope and it looks good (not 0x00).

Any Idea ?