Thanks for your reply, Rick.
We used TBV to keep track of motion in real-time in our experiment; I remember in general we had very little motion. We already excluded a few subjects that seemed to move too much.
I did a quick comparison of TSNR between a few "good" and "not so good" subjects. I saw a difference, the lowest was ~65, the highest ~89 (still kind of low compared with what Robert said).
In afni_proc, we used the default value of differential movement 0.3, and in most runs, we were able to keep 99-100% of time points.
Could the low TSNR be because of our task that didn't generate a high functional contrast?

Thanks,
Duong