engine does not turn over everytime with key

1974 Duster problem I have replaced the NSS, and ignition switch the starter relay but it still only starts intermittently with ignition switch. The car will start everytime at relay and just recently when the ignition switch is turned on the starter seems to be low on voltage and not engaging the drive gear if you jump the relay it works great. The car has the wires going to the seatbelt interlock switch but none of the seatbelt wires are attached in the interior. Any help would be great!