If you only think about that as the reason you've failed, then you have to think further.
It's not 'no signups' nor 'players died off' which caused the RPs to die. It's what made them happen. Why do people not join you? Why do people die off or lose interest in your roleplays?
You see, the problem of 'not having enough signups' happens all the time, even in some better Roleplays. 4 people would be enough to succeed, so it depends on other factors. People die off? Well some people 'die off' and leave some RPs, but remain in some others. It happens. Why do people lose interest in yours yet become so devoted in others?
If there's a problem bothering you, trying asking yourself why again and again.
Why do I say that I fail?
Because my RPs never finish.
Why do they never finish?
Because people aren't devoted enough and only a few amount of people joined.
Why are they not devoted enough? Why did those problems happen?
Because maybe the plot isn't interesting...
Why isn't it interesting?
Because I haven't added and twists nor encouraged them to continue with the RP while guiding them as GM?
Etc etc...
Soon you'll reach the core of the problem. I tried that once by asking myself and it helped (: