Maybe check the clutch faces themselves arent worn out. My last clutch was a bit slippy when pulling away and when i looked i only had 2 clutch surfaces left instead of the 3 its meant to have. One face had come off completely and was nowhere to be seen, just had the metal,i guess it just disintegrated into dust or something.
Maybe your new belt just needs a few miles to loosen up and settle itself in?