One of several things probably happened
The event you are playing is not a Daily Event
Daily Events have a day of the week in their names (e.g., "Tuesday Trap" or "Friday Blitz"). If it's not a Daily Event, it doesn't count toward unlocking the boss.
You already have a gold egg from this event
Maybe you earned it earlier today. Maybe you earned it several weeks ago (and maybe you forgot about it). Regardless, once you've been credited for it once, you can't get credit for it again. To progress in unlocking the boss, you should instead play Daily Events in which you have not yet earned the gold egg.
- Before you start the event, look just under the event. Next to your high score it will show the kind of egg you've already earned. If it's already a gold egg, then playing the event again won't advance your progress toward unlocking the boss. But if it looks like either of the pictures below then earning a gold egg will advance your progress toward unlocking the boss.
You earned the gold egg in the wrong chapter
Gold eggs only count toward unlocking the boss if they are from the same chapter as the boss. E.g., gold eggs earned in Chapter 2 don't count toward unlocking the Chapter 7 boss.