活动在后台时，Android服务无法连接到服务器

这是我关于stackoverflow的第一篇文章，所以希望我能正确地进行所有操作。我正在开发我的第一个android应用程序，并且在某些设备上遇到奇怪的行为。我有一个前台服务，可以定期获取设备的位置，并使用带有延迟运行程序的处理程序将其发送到服务器。在我的手机（棉花糖，API 23）中，一切正常，但是当应用程序的活动处于后台时，使用运行Lollipop（API 21）的小米手机的朋友无法连接到服务器。这是将位置发送到服务器的功能：

    private void sendLocation(final String request_id, final String location ) {
    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl(RestServiceConstants.BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build();
    String email = prefManager.pref.getString("UserEmail","");
    String password = prefManager.pref.getString("UserPassword","");

    Log.d(TAG,"sendLocation called");

    RestService service = retrofit.create(RestService.class);
    Call<StatusResponse> call = service.location("Basic "+ Base64.encodeToString((email + ":" + password).getBytes(),Base64.NO_WRAP),
            request_id,location);
    call.enqueue(new Callback<StatusResponse>() {
        @Override
        public void onResponse(Call<StatusResponse> call, Response<StatusResponse> response) {
            Log.d(TAG, "onResponse: raw: " + response.body());
            if (response.isSuccess() && response.body() != null){
                Log.d(TAG, "The location has been sent successfully");
                if(resendLocationHandler != null)
                    resendLocationHandler.removeCallbacksAndMessages(null);
            } else if (response.code() == 401){
                Log.i(TAG, "onCreate: User not logged in");
                prefManager.setIsLoggedIn(false);
            } else {
                Log.i(TAG, "sendLocation Unknown error occurred");
            }

        }

        @Override
        public void onFailure(Call<StatusResponse> call, Throwable t) {
            Log.i(TAG, "sendLocation Failed to get the server");
            if(resendLocationHandler == null)
                resendLocationHandler = new Handler();
            resendLocationHandler.postDelayed(new Runnable() {
                @Override
                public void run() {
                    sendLocation(request_id, location);
                }
            }, resendFailedRequestDelay);

        }
    });
}

我不知道我还能提供什么来帮助您诊断问题，因此随时要求提出任何可能相关的内容。提前致谢

编辑：请求失败的意思是它触发了onFailure回调。回调中捕获的异常是：

java.net.ConnectException: Failed to connect to my_server_address:80
    at okhttp3.internal.io.RealConnection.connectSocket(RealConnection.java:139)
    at okhttp3.internal.io.RealConnection.connect(RealConnection.java:108)
    at okhttp3.internal.http.StreamAllocation.findConnection(StreamAllocation.java:188)
    at okhttp3.internal.http.StreamAllocation.findHealthyConnection(StreamAllocation.java:127)
    at okhttp3.internal.http.StreamAllocation.newStream(StreamAllocation.java:97)
    at okhttp3.internal.http.HttpEngine.connect(HttpEngine.java:289)
    at okhttp3.internal.http.HttpEngine.sendRequest(HttpEngine.java:241)
    at okhttp3.RealCall.getResponse(RealCall.java:240)
    at okhttp3.RealCall$ApplicationInterceptorChain.proceed(RealCall.java:198)
    at okhttp3.RealCall.getResponseWithInterceptorChain(RealCall.java:160)
    at okhttp3.RealCall.access$100(RealCall.java:30)
    at okhttp3.RealCall$AsyncCall.execute(RealCall.java:127)
    at okhttp3.internal.NamedRunnable.run(NamedRunnable.java:33)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
    at java.lang.Thread.run(Thread.java:818)