Shift Stalling at 5&6

[Batfink]

I agree with Steve as in have you adjusted foot levers to suit you? I had to drop the relative gear lever about 8mm in total to help stop false neutral between 5th and 6th... There is a slight increase in gear lever action as you go up in the gears but I put it mainly down to "lazy rider" syndrome lol meaning you are hitting cruise speed, bike is settled so your physical movements are more relaxed and less articulate.
Otherwise I wonder when was last service and how good is the oil cause old/thin oils can give gear change issues.

Hope this helps