I had a similar problem, how I solved it:
mgmt.getGraphIndex('my_index_name').getIndexStatus(mgmt.getPropertyKey("property_i_ised"))
==>INSTALLED
graph.getOpenTransactions() - > showed me all the open transactions.
graph.getOpenTransactions().getAt(0).commit() -> for every open transaction
mgmt = graph.openManagement()
mgmt.updateIndex(mgmt.getGraphIndex('my_index_name'), SchemaAction.REGISTER_INDEX).get()
mgmt.commit()
mgmt = graph.openManagement()
mgmt.getGraphIndex('my_index_name').getIndexStatus(mgmt.getPropertyKey("property_i_ised"))
==>REGISTERED
