Do you have monthly meetings that you require or ask the entire PTO/PTA membership to attend? I see this topic of low attendance brought up a lot and I've never asked how often people meet.
This is what we do...
We have a Board that meets monthly. This is the President, VPs, Treasurer, Secretary, Committee Chairs (Hospitality, Volunteer, Staff Appreciation, etc.) We meet with the Principal and Assistant Principal as well since they serve on our Board. We have about 12 board members. During this meeting, we discuss items that have come up since the previous Board meeting, make plans, give committee reports and all the regular business that needs to be discussed. This is once a month and all the board members are expected to attend.
Then 4 times a year (2 each semester), we have General Assemblies. This is when we invite the entire membership and everyone else to the school for a "group" PTA meeting and vote on issues that the board recommends, etc. But we always plan these around BIG events that naturally draw parents up to the school.
In September it's Open House. We have our PTA meeting, then dismiss to visit the classrooms.
In December, we have the Class performance (for us it's the 3rd grades production of the Nutcracker). We have our meeting, then the musical begins.
In February, we'll have a Family night - last year was Arts and Crafts (right before Valentine's Day) - this was the first time we did this and it was a huge success. At this meeting we announce our nominating committee and begin searching for the the next year's board. After our meeting, we made crafts - but we've done game nights, and Fitness Nights, anything that draws a crowd.
In April, we have another musical - this time it's usually the 5th graders who perform. We have our meeting (elections this time) and then we have the performance.
When a Parent group must have a quorum, it's imperative you have an event that draws people up to the school. Not everyone can commit to serving on the board and meeting monthly, but 4 times a year is not asking too much. We have hundreds come out to these events and are always able to conduct business and vote on what we need to do because enough PTA members are there. Even though not everyone there is a member, they can still see what the PTA is doing for the school (by hearing it in the meeting and by participating in the activity.)
We also have many other events throughout the school year that do not involve having a PTA meeting before it. We do several Movie Nights, a carnival, a Talent Show and other Family Nights. These are held on Fridays or Saturdays. We also have several events that are during the day that the PTA hosts, but anyone can attend.
Anyway...that's my two cents.