take a lot of DC current (amps) to turn over. Corrosion on the battery terminal connections (even if they look good), issues with the battery to body ground cable that's right behind the battery (wire known to frey at the body lug behind battery), starter +12v wire not connected securely, etc.
Need to check all these things out - fairly easy to do. It's also possible the ignition switch has gone bad but check the other things first.
PS - If starter installed was a rebuilt (and not brand new) that could be the issue too?