Make sure that the two components are made to be assembled together:
Measure the ID of the coupling flange bore and measure the OD of the stub shaft. Then, if they are equal, verify that the spline count is the same for both. Finally, if there is a surface that is machined flat on the shaft, check that it is the same width as the corresponding flat in the flange bore.
How far - if at all - can you get the shaft into the flange bore? Did you check for foreign objects in the bore, distorted splines in either part, that the shaft is true, etc?