try vector obs

Project/Assets/ML-Agents/Examples/PushBlock/Prefabs/PushBlockCollabArea.prefab

   m_Name: 
   m_EditorClassIdentifier: 
   m_BrainParameters:
-    VectorObservationSize: 0
+    VectorObservationSize: 1
     NumStackedVectorObservations: 2
     m_ActionSpec:
       m_NumContinuousActions: 0
   m_Name: 
   m_EditorClassIdentifier: 
   m_BrainParameters:
-    VectorObservationSize: 0
+    VectorObservationSize: 1
     NumStackedVectorObservations: 2
     m_ActionSpec:
       m_NumContinuousActions: 0

Project/Assets/ML-Agents/Examples/PushBlock/Scripts/PushAgentCollab.cs

             ForceMode.VelocityChange);
     }
 
-    // public override void CollectObservations(VectorSensor sensor)
-    // {
-    //     if (useVectorObs)
-    //     {
-    //         sensor.AddObservation(StepCount / (float)MaxStep);
-    //     }
-    // }
+    public override void CollectObservations(VectorSensor sensor)
+    {
+        if (useVectorObs)
+        {
+            sensor.AddObservation(StepCount / (float)MaxStep);
+        }
+    }
 
     /// <summary>
     /// Called every step of the engine. Here the agent takes an action.