I have points on this from a couple of different perspectives. First of all, if the kids are happy with the status quo, they won't change their habits. If a kid was number four guy last year and got his varsity letter without training in the off-season and that is all he wants, he won't suddenly decide to train in the off-season just because the coach wants him to. He has to decide he wants more or feel like he is losing something if he doesn't.
Someone had mentioned meeting individually with the athletes to work with them on goals. That is a great way to figure out who really wants to put in the work and who doesn't. If they start talking about doing bigger and better things than what they've done in the past, then they are more open to working more to achieve those things. If they are satisfied with the status quo, they won't change.
If you get a couple of guys who decide they really want it, they will shake up the status quo. If your #4 guy hears that the #6 and #7 guys are training hard over the summer, #4 will either start working harder or you'll realize that he just isn't that motivated. By fall, the previous #4 guy will be your #6. It's better to not have to not have to count on a non-motivated guy to score for you.
About 20 years ago, our XC team won our conference but graduated numbers 1, 2, 3, and 7. In addition, our #4 decided that pot was more important than running and wasn't coming back out. Our best scenario at that point seemed to be a mid-range finish in the conference. However, we had one athlete who took it upon himself to make the team the best it could.
This athlete was a junior to be who had run around 19:00/11:15/5:10 as a frosh and 18:30/10:58/4:58 as a sophomore. After his freshman year, he said he wanted to be a state qualifier the next year. I gave him a reasonable plan and he barely ran a step over the summer. That fall, he was sixth man on our conference championship team. The next spring, after not getting the results he wanted that year, we talked about his goals. I have him a training plan again and he followed it to the letter, starting at 30 miles per week and working up to 50 by the end of the summer. That year, he ran 16:33 and was a state qualifier.
More importantly, he decided to step up and be a leader for the team. He inspired our top returner (#5) from the year before actually trained some over the summer for the first time in his career, dropping his best time from 17:30 to sub-17. Also, this leader recruited guys from the track team who hadn't run XC in the past. Every morning in the summer, he would leave his house at 7am and start picking up 4-5 guys to go running. Included in that was a freshman who was really hungry to win and wanted guidance. Our team ended up winning our conference again and just missed qualifying for state as a team (the three teams from our region were all top ten). At that point, the culture had been established for summer training.
Another option you may need to look at (although you probably don't want to) is to go to another school. Most coaches are very loyal to their schools (and especially the kids they coach). However, if you don't feel the conditions are right for you to build the kind of program you want, you have the right to leave. That doesn't mean hopping from one school to another every year or two.
I know a couple of coaches who were dealing with issues like yours. After a few years of being frustrated, one coach left for another school where he was able to coach both XC and track (he previous just coached track). Within five years, his XC team was top ten in the state and his track team was contending for the conference and regional championship every year. The other coach stayed at his school and five years later, he was still complaining about the same problems.
Good luck in improving your situation.