Your RP is just less than 24 hours since it was created. Give it time. And I don't know about the rule of when to bump, but most that I saw is usually one per day and after 24 hours since the RP- Interest Check is created. 

Be more descriptive with your RP as well. What do you mean when some advancements had been made? Are we referring to a new type of agricultural advancements, new energy sources, or new weapon types? The population is also an important factor when playing a zombie game. I did read that you said, "Set in the US." But what place do you have in mind? Is the RP gonna begin at the Washington DC during the winter or the Nevada Desert during summer? 

And maybe a quick tip. RP do need some emotions between characters as it not only spark something humane about them but also some drama necessary that would pull the RPers to continue. So you might want to change that to "action-based with soft spots" or something along that line.

And one final thing. If the RP does not gather any interest, don't worry. Just put it up and try it next time.