You can't elect a unfranked divvy if a company is paying a 100% franked dividends. Some companies pay both franked and unfranked but the company itself determines the percentage of each. If it is not paying any Australian company tax you will receive unfranked dividends.
Some companies pay a partially franked dividend with remainder being unfranked. Depends if they are an international corp and declare some of their profits were achieved outside Australia. That means no franking is paid on the proportion of profits earned outside Australia and paid out as dividends because the company doesn't pay Australian tax on that proportion.
Sorry if I'm repeating myself or failing to give you a satisfactory answer,
