Number 1 killer is the rampant ADD with the second reason being just a big enough slowdown to kill any momentum built up. (not counting advanced which moves on scales only known to tectonic plate shifts) 

I find players to tend to be on the passive side so there's like a cascade effect of no one doing anything until someone else does which stops things fast. 

Conversely just by keeping them caring engaged and whipping them into action you can like reverse psychology them into getting them moving a bit and hopefully get stuff rolling again. However if you lost interest than it should be obvious that you need to look at what isn't fun about it and change it or just face it that it's not fun and let thing crash because well tough shit.