I haven't looked at any of the data sheets in a while, but if I remember correctly, when I was using the DataFlash AT45 parts way back when, there is a clock polarity issue - you will have to play around with the clock polarity/phase on the master SPI port to match. So, if the AT25 parts use the same SPI hardware, you may face this as well.
Again, I'm not 100% certain of what's going on, but I thought I'd pass on my findings from a while back.
b