Many great ideas already, but I'll add a few more.
The main thing you need to keep in mind ---- You already know that 6-10 pm works. Be sure to not loose sight of what makes that work as you try for more up-time.
The second thing to remember ---- Don't make this a do or die thing. If it works, great! If not, don't become discouraged. You still have your core staff and club members that already have stuck with you over time.
Start slow.
Perhaps make announcements and plan for earlier starting times for a dance/set and also after hours dancing. Be sure to have staff for both. Having DJ's that play a variety of music might be a good idea as well. If your club plays mostly Rock, there are varieties of Rock music that could be played, such as classic rock, soft rock, hard rock.
Check with the current members and staff to see what they think and if they have any ideas that might help. You wont be able to use every idea, so think them over and see if they fit in with your goal.
Lastly, you can't MAKE it happen. No amount of money, time, and resources can quarantee a club will become popular. Or in your case, more popular. If you've been in SL long enough, I'm sure you've seen clubs pop up, claiming to be (or soon to be) THE PLACE TO BE IN SECOND LIFE! Before long, the club is gone.
Be patient. And best to you in your venture.
Bear