Call them and ask them if they will sign them for you - I had to do this with my Paint mare. The person(and a few before them) who I bought her from never transferred her so I had to track down the woman who last owned her and she happily agreed to sign them and sent them back to me.
If they refuse just call the registry and explain to them you have the papers and a bill of sale but they refuse to sign. They should do something for you, but every registry is different I suppose.
Some days it's not worth chewing through the straps.