Go back to basics - check that battery, cables, and connection are all good. If so, then replace the solenoid again (make sure you are using a NEW solenoid, the high current contacts in the solenoid are the primary failure point in almost all nippon-denso starters). Do not assume that the battery is good just because no-load voltage is OK, please test that voltage does not drop below 9v when a 300 amp load is applied.