2002 Manual Transmission Shifting Issues

[RPM23]

2002 Ford Ranger 4.0L V6 4x4 5 Speed Manual: Cannot shift into gear regularly/periodically when engine is on. Shifting when engine is off is easy, with rare binding or getting stuck but generally easy. While engine is on, shifting up to other gears and back to first sometimes allows it to get into gear, but sometimes all fears are locked up and engine needs to be turned of and then shifted into first before restarting the engine and driving. Clutch has engagement, the rear has been jacked up and shifted into first and there was no lag when clutch is released from the floor, no suspected air in the system, but the fluid may be old. When car has been driven for a while or is warm, little to no shifting issues. What could the possible issues be and what are the solutions? New ATF mercon 5 was drained and refilled ~6k miles ago