During my first week driving car(2019 VN with Performance Pacakge), I too was struggling to get into 6th gear. However, as time went on I noticed that if I waited until tacometer indicated that It was time to shift up that was when I was able to shift into 6 with ease. If I shifted from 5 to 6 a few RPMs early, the gearbox would prevent me from shifting into 6th gear(no grinding noise, it just sits there but does not go into 6th). Im not sure if I should bother Hyundai about it or just change the Transmission fluid.