I have had some tiles that will not cut on a snap cutter, no matter what you do, years ago I was tiling a basement floor and was setting 8x8" (200mm) tiles.
The floor was over 600 square feet diagonal set, every single cut needed to be cut with a wet saw.
One thing you might try is to use less pressure to score and not ever score twice. The surface finish will also make a big difference, high gloss will cut better than rough (stone-slate) finish.
How long is that tile? You might need a bridge saw to cut them.