Which oil have you got in the primary?
Asking this because some time ago I had the same kind of problem due to a to thick oil in the primary leading to a sticky clutch. Changed it to an even thicker one. Don't ask me why I did this because it only made the the clutch more sticky, but it seemed a good idea at the time...
Since then, cleaned the clutch plates and changed the oil to an SAE 10 grade. Switches gear as through butter now.