Map.get(key) returns null after inserting key

Hello,
I am inserting a key and a object into a Map. later in the code I try
to retrieve the object by calling
Map.get(key) and I get a null.
I can't see where is my problem...

The application is a weka classifier.
the problem can be seen in isAcceptable(Instance) function.

First I populate a hashtable (calculateAprior method) where the key is
an instance and the value
is a double.
later I will try to fetch the double value by the key and get null (in
isAcceptable method).
I will eventually recieve a NullPointerException.
Thanks

CODE:
/*=============================buildIB3TrainingSet===========================*/

 private Instances buildIB3TrainingSet(Instances instances) {

          /** see implementation below <==**/
      calculateAprior(instances);
      double distance
=0,minDistance=Double.MAX_VALUE,classValue=Double.NaN;

      /*** Sucess rate*/
      successRate = new HashMap<Instance,int[]>();

      Instances res = new Instances(instances,0,1);
      this.addSuccessSuccessRate(res.firstInstance());
      //Iterate on original
      for(int i=0;i<instances.numInstances();i++)
      {
          Instance candidate = instances.instance(i);
          Instance yInstance = null;
          //Find nearest neighbor
          for(int j=0;j<res.numInstances();j++)
          {
              Instance conceptDescription = res.instance(j);

                         /** see implementation below <== here is the
problem!!!!!!!!!!!**/
              if(!isAcceptable(conceptDescription))
                  continue;
              if (!conceptDescription.classIsMissing())
              {
                  distance = distance(candidate, conceptDescription);
                  if (distance < minDistance)
                  {
                      minDistance = distance;
                      classValue = conceptDescription.classValue();
                      yInstance = conceptDescription;
                  }
              }
          }
          if (yInstance!=null)
          {
              if(candidate.classValue()!=classValue&&classValue!=Double.NaN)
              {
                  res.add(candidate);
                  addFailureSuccessRate(yInstance);
              }
              else
              {
                  addSuccessSuccessRate(yInstance);
              }
          }

          for(int j=0;j<res.numInstances();j++)
          {
              Instance conceptDescription = res.instance(j);
              if(isRejected(conceptDescription))
              {
                  res.delete(j);
                  successRate.remove(conceptDescription);
              }
          }
      }
      return res;
  }

/*=======================calculateAprior===================================*/
/**
 * calculateAprior
 *
 * @param instances
 * @return double array of aPriory
 */
private void calculateAprior(Instances instances) {

    this.aPriori = new
Hashtable<Instance,Double>(instances.numInstances());

    /** count classes*/
    Map<Double,Double> classCounter = new
HashMap<Double,Double>(m_Train.numClasses());
    for(Enumeration enumeration =
instances.enumerateInstances();enumeration.hasMoreElements();)
    {
     Instance ins = (Instance)enumeration.nextElement();
     if (classCounter.containsKey(ins.classValue()))
     {
     classCounter.put(ins.classValue(), new
Double(classCounter.get(ins.classValue())+ 1));
     }
     else
     {
     classCounter.put(ins.classValue(), new Double(1));
     }
    }

    /** Update instances a priori*/
    for(Enumeration enumeration =
instances.enumerateInstances();enumeration.hasMoreElements();)
    {
     Instance ins = (Instance)enumeration.nextElement();
     aPriori.put(ins,
     (double)classCounter.get(ins.classValue())
     /instances.numInstances());
    }
}

/*=========================isAcceptable==================================*/
/**
 * is the Instance acceptable
 * @param j
 * @return
 */
  private boolean isAcceptable(Instance instance) {
      int [] inf = successRate.get(instance);
        double rate = (double)inf[1]/inf[0];

               /* here is the problem!!!!!!!!!!! aPriori.get(instance)
is null!!!!*/

        return (rate-aPriori.get(instance)>ACCEPTENCE);
}