The thinner the paper, the lighter the score you must make. (Not "less wide", but less impression.) Remember that generally you are breaking the paper fibers somewhat when you score (or crease), even if it looks as if you are simply compressing them or re-forming them.
A sheet that has been baked in a digital machine will be more brittle and won't be very supple until it has absorbed some moisture. And even then it won't approach the suppleness of a sheet that never was baked.
So: Try to let the sheets re-humidify a bit and use the broadest score you can get away with, with the least pressure. If you can channel score to give 2 "hinges", do it.
As a general rule: 100# gloss book and lighter simply needs a nice, loose fold along the grain without any score at all. If you want a tight fold, that's where you'll get your problems.
If anyone has scored on Letterpress they will know the answer. Typically scoring will look better against the grain. The fold line is then defined. With the grain not so much. In both cases you can typically achieve acceptable standards but leave no doubt scoring against the grain yields a nicer fold line...... That's been my experience anyways....
One the letterpress try using .900 rule when scoring across the cylinder and .895 going into the cylinder. Also sometimes 1pt rule (you will need proper matrix to run this) will work better when scoring with the grain and 2pt when scoring across the grain. This is assuming your packing is set for .918 cut and not .937. and running 100# text. Also when scoring across the cylinder, after you run 5 or so sheets look at your matrix and make sure your rule is not making contact with the shoulder. If it is, you will notice one shoulder of the matrix will be slightly smashed.