It could be lots of things.
What keywords are you targetting? Some keywords get 0 traffic, others get hundreds per day. But keep in mind the more traffic a keyword gets the more competition there is for that keyword. If your new DON’T try to rank top10 for “online casino”, “online casinos” or “online gambling”.
If your sites aren’t ranking well the only thing that can boost them (assuming they’re optimized correctly) are more and more links. IMO quality links are the only thing that can theoretically boost your site infanantly.