Are you selecting them individually to change threshold or altogether? Try the individual selection if you haven't already.
Others have said they have had issues in the past with transient detection but it's not something I've experienced yet.
In my experience Audiosnap always works better after a clip map has been edited, even if it's just setting the tempo correctly so that may be worth checking as well.
If none of that helps manual transients can be added by Alt clicking.
HTH