You should ensure that the "INDEX" field is part of the subscription schema.
This cannot be determined by the provided code snippet, as the
new OverwriteTable(null, null, "MERGE");
form (with no schema specified) implies that the Html Client Library finds the fields by looking for your Html cells in the page.

You can easily check which fields are subscribed to by looking at the Server log.
You can find them embedded in the "/lightstreamer/control" request, logged with the factory log configuration;
alternatively, you can have a more detailed log by setting the "LightstreamerLogger.subscriptions" logger at DEBUG level.