Apache Kafka (KStreams) : How to subscribe to multiple topics?

I have the following code

//Kafka Config setup
Properties props = ...; //setup

List<String> topicList = Arrays.asList({"A", "B", "C"});

StreamBuilder builder = new StreamBuilder();
KStream<String, String> source = builder.stream(topicList);

source
.map((k,v) -> { //busy code for mapping data})
.transformValues(new MyGenericTransformer());
.to((k,v,r) -> {//busy code for topic routing});

new KafkaStream(builder.build(), properties).start();

My Problem : When I add more than one topic to subscribe to (ie A,B,C in above) the Kstream code stops receiving records.

References : https://kafka.apache.org/10/javadoc/org/apache/kafka/streams/StreamsBuilder.html

Relevant Documentation

public <K,V> KStream<K,V> stream(java.util.Collection<java.lang.String> topics)

"If multiple topics are specified there is no ordering guarantee for records from different topics."

What I'm trying to achieve : Have one Kstream (ie 'source' from above) consume/process from multiple topics.