JpaRepository findOne(id) returns null

This is my first post here, I've been searching for a long time here but I didn't found a problem that seemed similar.

When I use JpaRepository function findOne(id) for one of my classes, it returns null. As if no row had been found for this id. Of course the database row with this id exists.

Also my class mapping seems right.

I don't understand because I already used findOne() for other classes and I never had any problem.

Anyone can tell me what can be the source of this problem, please ? That would be nice !