The voter turnout have more approach than just put a mandatory.
The electoral approach: The electoral body often make some advertising, promoting the use of vote, specially in young democracies. Making the vote mandatory is a way but I think there are other ways to increase it such as promotion of more transparency in the electoral process, allowing independent parties to seat in the polling table (juridically and electorally), increase the participation of people with disabilities and others.
The political approach: The electoral body put all the tools to make possible the vote but the one who cheer people to participate is the politicians. People only react to a certain political spectrum, meaning that participation also depend of what the political parties have to offer to citizenship. We could make some comparisons between a local and national election: the local election has a certain participation and the national other. Why? Because the promotion of each election; There's also the hypothesis that political parties (the big ones) don't want lot of participation because they don't have all the votes.
The society approach: When the society is weak, when the civic education is low (or doesn't exist), when there's no promotion of real democracy, society won't participate because they're not interested or they don't know the advantages of the process. This is when NGOs such as International IDEA, UN Political Office (in their Electoral Department) and other organizations have an important role helping with electoral assistance through the electoral body.