We consider the two-dimensional steady flow, down a semi-infinite straight channel, of a liquid of viscoplastic type (i.e. exhibiting a yield stress), in the region far enough downstream from the entrance that the flow is almost parallel. It is shown that a paradox arises if the rheological description of the liquid is taken as the simple Bingham model, according to which the unyielded material is absolutely rigid. The difficulty is removed by relaxing this model, using the Biviscosity model instead. The approach to parallel flow by means of decaying eigenfunctions is calculated. It is shown that, as the Biviscosity model approaches the Bingham model, this approach becomes infinitely delayed.