Are you using any mods?
Unemployment doesn't matter, I've had lots of unemployed sims take part in both contests. It also doesn't matter if your sim has a job. You can go after work (schedule permitting), work from home, take a vacation day, take a sick day, leave work early, skip work entirely, etc. None of that should make any difference. The Hackathon and UGT should start about an hour into the festival and end about an hour before the festival does. That's your window. If that's not happening, you either have a mod conflict or a bug.
Edited to add: Weather can be a problem if you have Seasons. The computers, gaming mats, and other electronics won't work if it's raining and will break pretty quickly. Snow probably has the same effects.