Media Player is playing get confused to catch Custom Next Intent

Question

I'm made simple Media Player by using MediaResponse .

Google automatically handle : play , pause , stop , resume .

Since Google not supported : next function yet, I created custom Next Intent to handle next function.

I had 2 cases makes me confused after talk to Dr. Media then go :

MediaResponse show correctly, not playing yet (not click button Play yet).

Case 1 - Then I say next (or next song ), it matched with my defined phrases in my custom intent. And Next function is OK.

Case 2 - I clicked Play button to play audio , then I say next (or next song ), it didn't match with my defined phrases (it oftens say Okay and nothing happens), so Next function is not OK. (As images in below)

In case 2, How I can catch my phrases next , next song , play next ... when Media Player is playing? ( In other words, these phrases can not trigger my custom intent )

Please help me, Thanks

p/s : It happens on Mobile phones

actions_intent_NEXT.json

    {
  "id": "01055a59-26a0-4f39-a770-6bc5404482d9",
  "name": "actions_intent_NEXT",
  "auto": true,
  "contexts": [
    "actions_capability_screen_output",
    "actions_capability_media_response_audio"
  ],
  "responses": [
    {
      "resetContexts": false,
      "affectedContexts": [
        {
          "name": "actions_capability_screen_output",
          "parameters": {},
          "lifespan": 5
        },
        {
          "name": "actions_capability_media_response_audio",
          "parameters": {},
          "lifespan": 5
        }
      ],
      "parameters": [
        {
          "id": "2ce8514c-a346-4251-9a35-cab32d2c9d7c",
          "required": false,
          "dataType": "@nextPlay",
          "name": "nextPlay",
          "value": "$nextPlay",
          "isList": false
        }
      ],
      "messages": [
        {
          "type": 0,
          "speech": []
        }
      ],
      "defaultResponsePlatforms": {},
      "speech": []
    }
  ],
  "priority": 500000,
  "cortanaCommand": {
    "navigateOrService": "NAVIGATE",
    "target": ""
  },
  "webhookUsed": true,
  "webhookForSlotFilling": false,
  "lastUpdate": 1542252675,
  "fallbackIntent": false,
  "events": [
    {
      "name": "actions_intent_NEXT"
    }
  ],
  "userSays": [
    {
      "id": "e28a4087-2bf9-494d-9790-c7347f870ee4",
      "data": [
        {
          "text": "next song",
          "alias": "nextPlay",
          "meta": "@nextPlay",
          "userDefined": true
        }
      ],
      "isTemplate": false,
      "count": 0,
      "updated": 1542249008,
      "isAuto": false
    },
    {
      "id": "7a8cf2a2-d131-490d-977d-1212f4642f52",
      "data": [
        {
          "text": "next",
          "alias": "nextPlay",
          "meta": "@nextPlay",
          "userDefined": true
        }
      ],
      "isTemplate": false,
      "count": 0,
      "updated": 1542248700,
      "isAuto": false
    }
  ],
  "followUpIntents": [],
  "liveAgentHandoff": false,
  "endInteraction": false,
  "templates": []
   }