Make sure the plates are flat, which isn't always the case, even when new.
Then, make sure that the rod and linkage are properly adjusted, which sometimes takes a bit of trial and error work.
The clutch is an old style British clutch, and doesn't have much throw to disengage and engage, and anything that 's not quite right can make it drag.
And it cannot withstand sitting at traffic lights with the clutch lever pulled in.
That will overheat it and warp the plates, and it will get worse.
And it will wear the rod or friction-weld the ball bearing on the rod to the rod, when you hold in the clutch lever for a long period.
Put it in neutral when stopped at a light.
If you make sure that the plates are flat, and you play with the adjustments for a while, you should be able to get it.