I've run into clutch slip being temperature-sensitive on occasion ... Engines make more power and torque in cool weather, for example (because they're getting a denser air-fuel charge) and that favors clutch slippage. It's possible that your engine makes more power when it's warm enough to run decently but not completely up to operating temperature. (you have an air-cooled bike, they generally run hotter than we would "like" them to run)
In any case, the clutch in your bike should have enough capacity to handle these fluctuations. Maybe it did originally, and now it doesn't, which means something's up. Maybe the springs don't provide enough clamping force, maybe your release mechanism is binding/jamming and not fully releasing (been there, done that), maybe you didn't replace the right parts, maybe your clutch basket is so badly notched that it's not allowing the plates to fully engage. (Look at the slots in the clutch basket that the tabs on the friction plates engage with) Or maybe you put something together wrong - forgot a spacer, parts put together in the wrong order, didn't check stack height and select-fit plate thicknesses accordingly (usually this is for "slipper" clutches but check the service manual), etc.
On a yamaha fizzer, the pressure plate will go together 5 different ways, but only one of them works ... the one where the alignment marks on the pressure plate and the basket line up ...