New fibre plates? Are they from a trusted manufacturer and supplier? You wouldn’t be the first to be hampered by crappy pattern parts!
Tight primary chain can definitely cause this.
As can shit / degraded gearbox oil.
Lack of clutch lift is another prime suspect, check its adjusted properly (ensure cable is slack, then adjust clutch centre nut, then adjust cable), if the lifter mechanism is partially lifted by the cable before adjusting the clutch centre nut, you’ll loose lift, and get drag.
But possibly the most common reason on a Triumph is a pressure plate that is not lifting and seating perfectly square. This will cause both slip and drag.
And another possible cause is the cam plate plunger area. I’ve seen them where the housing fouls the cam plate because it’s screwed in too far. Also sometimes there is just too much pressure from the plunger and spring acting on the cam plate and fitting an extra washer or two to space it off helps tremendously. There are different types of plunger too, pointy vs rounded. Experimenting with different types can help. There are plungers available with a captured ball in the end which look like a really good idea but they’re NOT. They allow the cam plate to foul the none ball part of the plunger and just don’t work.
A 5 speed Triumph box is just about as sweet as a box gets when set up right, so if it’s not shifting nicely, something is wrong.
However... it is certainly not uncommon for them to be difficult to get into neutral at a standstill, you might be chasing a unicorn with that one !
I found Bel Ray made mine far smoother shifting than Shockproof oil BTW.
