Elsa,
Adding contrast as in the "Brightness and Contrast" is not going to work. Brightness and Contrast are "lousy" and throw out data. A curve is needed as in "Levels and Curves" as the brightest and darkest values you want in the print need to be saved.
When you use "Contrast" it will push values out the top leaving larger pure white areas. The most you want are tiny pure white areas such as specular highlights. Usually you want a hint of detail in the darkest parts of a print but "Contrast" will push this hint of detail out the bottom so you end up with large areas of pure black. "Contrast" causes clipping in both the highlights and shadows unless the original image was quite flat.
This might be the suspect:
https://www.instagram.com/p/BTF1fD9BKiE/http://russellbrown.com/...who said "Brightness and Contrast" was included in Photoshop for amateurs.
Instead of using brightness and contrast use "Levels and Curves." These are sometimes separate and sometimes combined depending on the program in use. The object is to keep generally keep the high and low points and modify the contrast in between. You generally want good mid tone contrast. A straight line from dark to light give a very flat, chalk and soot appearance.
Now wild guessing: I'm thinking you are having trouble with highlight contrast so perhaps it should be steep. You might draw down the 1/4 tone with one point in curves thus compressing the shadow contrast. The contrast will build giving useful contrast in the mid tones and even more in the highlights.
Printing profiles: I think this is something like matching a negative to a sheet of printing paper. I'm pretty sure they are applying a curve to image to fit it on the printing media in an attractive way.
A profile and soft proofing should take care of a lot of trial and error. It sounds like a printing profile is like the quest for the Holy Grail.
Best,
Dave Hartman
