Ok, well, I don't know much about Walsworth because they aren't big in this area, but at the national conventions, I've never heard anything bad. The most important thing you should be getting from them is support. If you are concerned about selling ads, then they need to send a rep into your school to help you out. If you are concerned about budgeting, then your rep needs to be there working out the budget with you, you know what I mean? They need to work for their money! LOL! I use Taylor Publishing, and I loved them, but honestly, each company is only as good as its rep.
Are you allowed to tie their sales quota to their grades? If so, you need to do that. In my class, meeting deadlines (and all the steps that led to it) was 50%, and meeting the sales quota was the other 50% of the grade. I can send you a specific breakdown of that if you want. Older/more experienced staff members had to sell more ads than younger/less experienced staff. Do you sell vanity or senior ads in your book? That was our biggest source of revenue. I'm assuming that the yb is part of the school's tuition, is that correct? So what you are really paying for is extra color and such.
Your book sounds exactly like my daughter's school's book-- k-12, private school... It is difficult to get the older kids to go to elementary events. Here's the first rule in yearbook production: THE BOOK IS THEIR BOOK NOT YOURS! Don't do a single page in that book! You have to be a Nazi about that, or you will end up burned out and completing that whole book on your own.
For the elementary kids, I would make an elementary editor whose job it is to contact all of the homeroom mothers of the different classes. Those ladies LOVE to take pictures of their little darlings. They'll be more than happy to supply you with pictures. You just need to give them a deadline for submission. You could even offer a "photo class" for them one day after school and teach the moms what kinds of pictures make good yb pictures. Trust me, they LOVE that kind of stuff. If possible, I would also see if you could get couple of 5th graders to volunteer to help with the book after school once or twice a week (or however often you need it). Let them help you identify the names of the little ones. They'll know who most of the kids are, and they'll like hanging out with the big kids.
As far as keeping them motivated... well, welcome to the world of senioritis! LOL! Seriously, the best motivator is a grade for most of these kids. I'll send you some stuff on how I grade if you'd like.
I'm droning on and on here.... I sent you a pm, too. As you can probably tell, I get a little excited when it comes to yb! I'll help any way I can!