jpeg attacks and watermarking?

I have a problem with watermarking in DCT domain. I divide image into 8*8 blocks and DCT transformed each block. when I embed with strength factor alpha={1,...,10}, in jpeg attack with quality Q=60 , Q=40, by increasing alpha the NC value decreased but in Q=50 , Q=100 the NC value increased, what is the problem? is it reasonable?

[nadia]

no one has any idea??!!