If both starters check out OK , ie with jump leads on main feed and strter body , when a feed is supplied to solenoid activation terminal then dog is thrown forward and stays there revolving .
The jump lead test is not 100% certain , as its does not test torque load , just that you are getting it to motor under no load.
The next thing is to look for possible problem with the solenoid feed.
This should be done with measurement under loaded (with solenoid engaged) using clip on ammeter. If not possible then , it is good enough in most cases to just check for 12v . when key turned, try wiggling key about to see if poor connection in key unit . HTSH