Ask HN: What is your biggest problem when organising an event?

My name is Kostas and I have been involved in the organization of an annual tech conference for the past five years, in Greece. The event has been steadily attracting ~500 attendees each year and is being run by a team of ~30 volunteers (avg). I've been one of them since day one and the event (and the company behind it) is non-profit.

I am always interested in talking with other organizers and the most common question I always ask is what is your biggest problem when organizing an event? Are there any sort of issues you keep bumping into? What are your thoughts?

Thank you,

Kostas