Scala Pickling: Some of the constructor parameters could not be serialized

Question

When I try to json pickle an instance of this class (using this line: cls.pickleTo(....) ), N, timeWindow and lastN parameters do not show up in the result. No problem with other parameters. Any tips for this problem?

class CluStream(dimC:Int, dimD:Int, NC:Int, ND:Int, N:Int, threshold:Double, timeWindow:Double, lastN:Int, uniqueLabelCount:Int=1){
    ........
}