Measure spring height. You don't need heavier springs, perhaps just newer springs will do?
Also warpage in the clutch plates could create the problem you're having. Lay the plates (after de-glazing) on a sheet of glass and try to slip a feeler gauge underneath. In a pinch you may use two sheets of 24lb paper. If the papers slip under the clutch plate at any point along its edge, replace the clutch plate. Otherwise, get a gauge that matches tolerances found in the bike's repair manual.