some common sense about fibre prints:
The paper side dries a lot slower than the emulsion side in air. The emulsion side dries a lot faster than the paper side. So when a print dries the emulsion shrinks and curls the paper. The paper side then dries but with all the fibres shifted so that it has a permanent curl.
Then what happens is you stick it in a press. But because all the paper fibres are still dry and springy, all you do is put a temporary flatness in it which will eventually move back to its curled state as that is where the fibres dried.
So the solution is, as bob carnie has already told you, to damp the print paper before putting it in the press. That way the fibres can move instead of just being sprung into a new position.
And from all of that you should also be able glean that drying fibre prints very slowly will cause less curl and drying them face down on screens will limit airflow over the emulsion side so that the paper side dries relatively quicker causing less curl. But best of all, drying them very slowly in blotters with a weight on them will dry them perfectly flat with no tension in the paper fibres at all. That way they will stay flat even if not dry mounted.
common sense really if you actually think about it.
If you are the impatient type then you are stuffed.