diff --git a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/NativeAlarmActor.cs b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/NativeAlarmActor.cs
index 168e40d5..526e6757 100644
--- a/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/NativeAlarmActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/NativeAlarmActor.cs
@@ -129,6 +129,15 @@ public class NativeAlarmActor : ReceiveActor
             SourceReference: _source.SourceReference,
             ConditionFilter: _source.ConditionFilter,
             Timestamp: DateTimeOffset.UtcNow));
+
+        // Arm a response-timeout retry so a LOST subscribe response is re-sent, not
+        // only a failure reply (S4). Cancelled on a Success reply; a failure reply
+        // re-arms it at 1× (overriding this 2× response-timeout arm). 2× interval so
+        // a slow-but-arriving response is not pre-empted by a redundant re-send.
+        _retryTimer?.Cancel();
+        _retryTimer = Context.System.Scheduler.ScheduleTellOnceCancelable(
+            TimeSpan.FromMilliseconds(_options.NativeAlarmRetryIntervalMs * 2),
+            Self, RetrySubscribe.Instance, Self);
     }
 
     private void HandleRehydration(RehydrationCompleted msg)
@@ -270,6 +279,8 @@ public class NativeAlarmActor : ReceiveActor
     {
         if (resp.Success)
         {
+            // Success — cancel the response-timeout retry armed in SendSubscribe.
+            _retryTimer?.Cancel();
             _logger.LogInformation("Native alarm subscription established for {Source} on {Instance}",
                 _source.CanonicalName, _instanceName);
             return;
diff --git a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/NativeAlarmActorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/NativeAlarmActorTests.cs
index fb37f76f..3fe305a9 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/NativeAlarmActorTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/NativeAlarmActorTests.cs
@@ -378,4 +378,18 @@ public class NativeAlarmActorTests : TestKit, IDisposable
             File.Delete(_dbFile);
         }
     }
+
+    [Fact]
+    public void LostSubscribeResponse_ResendsSubscribe()
+    {
+        // S4: a LOST subscribe response (no reply at all) must be re-sent, not only
+        // a failure reply. The response-timeout arm fires at 2× the retry interval.
+        var options = new SiteRuntimeOptions { NativeAlarmRetryIntervalMs = 200 };
+        var dcl = CreateTestProbe();
+        Spawn(CreateTestProbe().Ref, dcl.Ref, options);
+
+        dcl.ExpectMsg<SubscribeAlarmsRequest>();
+        // Swallow the response entirely — the actor must still re-subscribe.
+        dcl.ExpectMsg<SubscribeAlarmsRequest>(TimeSpan.FromSeconds(5));
+    }
 }