In the framework of the product ansatz as an approximation for the two-baryon system we review in detail the derivation of the isoscalar nucleon - nucleon spin - orbit potential coming from the sixth-order term of the extended Skyrme model. We show that the sixth-order term contributes with a positive sign, as is the case for the Skyrme term, contrary to the claims of Riska and Schwesinger. These authors considered only one part of the force due to the sixth-order term and omitted the second part which turns out to be the dominant one. Our result is independent of the parameters of the model. We discuss briefly the possible origins of this difficulty encountered by Skyrme-type models to predict an attractive force.