Old Milliput 'sausages' do feature some kind of 'crust', that will not mix properly and prevent it from curing. On the 'normal' Milliput, in a white box with red trimmings, this crust is mostly apparent on the grey sausage, the yellow remaining usually very soft. Cut throgh the sausage and see if the crust extend to the middle, in which case you can throw everything away. Most of the time, the crust is less than 1mm thick, so it can be shaved off and the good stuff mixed with the other component.