Some papers will not curl.

It is often best to judge by the smoothness of the paper rather than curl. I have the same problem.

Paper, when manufactured is produced with pulp or rag rising out of a slurry onto what amounts to a moving wire frame. This is the bottom of the paper and is called in the trade, the wire side. The other side is the finished side. After partial drying the paper is pressed between two calendaring rollers that squeeze out moisture. These rollers may be hot (hot press) or cold (cold press) and remove most of the roughness from the wire, but not all. Therefore, one side is always rougher than the other.

Afterwards, the smooth side is often coated with a sizing layer such as is done with COT320. Bergger uses gelatin for this.

That gel size can be heavy or light. If light, then the paper will not curl or will curl very little if you wet it for the type of curl test above.

PE