That's strange - my 45mm is an exceptionally sharp lens. Shallow DoF, perhaps? All fast lenses need stopping down for certain applications, but I'm sure you're aware of that. The 45mm does a very pleasing job at subject isolation - but that's not always what you want, of course. Stop it down to f/2.8 for even stronger IQ and a lot more DoF - it's a lens, after all. It works well down to f/5.6 without any signs of diffraction (f/8 is possible as well, just a tad less crisp, but almost as good in the center as wide open).My daughter has the 45, and it is fine for portrait use, but to my eye , it's a bit soft for my use.
The 30mm f/1.4 does even better at subject isolation at close quarters - and thus it also needs stopping down for compelling DoF, f/2.8 and f/4 are very useful and beautiful.