2000 Alfa Romeo 156 gearbox

Transmission problem
2000 Alfa Romeo 156 Front Wheel Drive Manual 170000 miles

hi, only when the engine gets warm (after aroun15 miles of driving the car) it does not go into gear. Changed oil in gearbox but still the same. Clutch seems fine to me and it has also been changed in the past 6 months. If you turn the engine off it goes in gear fine but after starting car again problem reoccurs. No other issues with it as I just got it serviced. Thank you