On your iPhone, go to Settings > Messages > Text Message Forwarding and make sure your iPad is enabled there.
Without doing this, only iMessage texts will appear on your iPad. You won't see anything from normal SMS texts.
You can also check out the send and receive address settings on your iPad in Settings > Messages. You can receive from several email sources and the iPhone's number (when enabled on the iPhone). You can only send from one address, so make sure the iPhone's number is selected.