Hi Siauhwa,
Typically a scope probe has a small capacitive-inductive-resistive load associated with it (it should be mentioned in the probe data sheet), that can alter the line characteristics, which is why touching a scope probe makes it work.
What is the configuration of SSI module for interfacing it to AD7324. I would ask you to check the following as well
1. Is the SSI CLK frequency less than or equal to the maximum supported by AD7324
2. Is the setup time of CS and/or DOUT of AD7324 w.r..t CLK at the AD7324 being met(you may want to use an active probe to reduce line loading)
Regards
Amit Ashara