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In the mechanism of Hofmann rearrangement, one step involves migration of an R−group from carbon to nitrogen.
This reaction proceeds even if the migratory aptitude of the R-group is poor (like in the case of ethanamide). What motivates the migration of the R-group and makes the reaction proceed?
The ‘motivation’ is that migration is the only way in which the reaction proceeds in a productive manner towards the product. From seeing the mechanism alone, you have no idea if the reaction of a particular substrate is rapid or slow, if it requires higher or lower temperatures etc. Only if you start taking these into account, you can tell whether the compound in question reacts rapidly or not.
I would not be surprised to see vastly different reaction times between compounds, depending on whether the $\ce{R}$ group easily migrates or not.
