How do you generate enthusiasm?

Last night, we had our monthly meeting for MKEPUG (Milwaukee, WI). We didn't have high expectations, since only 13 people RSVP'd, but we only had 5 show up. The topic was new items in PHP 5.4 and 5.5, something several members had expressed interest in. To make it even more engaging, we formatted it as an interactive discussion, instead of a single speaker talking for 45 minutes.

Overall, we have a core group of regular people, but our attendance averages 15 for a group of over 300 members. We'll occasionally spike up to 25 or 30 if we have a visiting speaker, but that's rare.

I'm discussing the issue with my fellow organizers and my current plan is to send a heartfelt letter to the members asking them why more people don't come? Is it logistical (bad time/location/day)? Is it the topic? The format? Something else?

We even have free beer (and soda/water/juice) provided by our host! The location is comfortable with free wifi, plenty of seating, good A/V, free parking.

I'm getting frustrated because I want a more vibrant community. Any advice/suggestions/help you can offer would be greatly appreciated.

Joel Clermont