jas2k06

So i just got my Clutch replaced 400 miles ago and today on the highway it started slipping when i was in 6th gear over 3500 Rpm and been driving it for a good 30mins but it only happens when the car is warmed up, if its cold it works great but after awhile the clutch gets stiff and it slips in 4th 5th and 6th gear over 3500 Rpm
Do u guys have any idea why this is happening ? any Input would be greatly appreciated.

Its got new Slave cylinder and master cylinder and a resurfaced flywheel