welcome to the #1 complaint of 43.xx lol... we use to be able to force attacks... that option is currently broken.
cerevox is fairly correct. basically all civs send out scouts if a scout spots a fortress he escapes the map and runs home, if your fort meets civ criteria for assault they send out an invasion/siege force. that group has to travel to you. after each unsuccessful attempt, a new force is created and sent out at the time their race sends out such things... each new assault group is suppose to be harder then the last.. but they only move during the season they are designed too. If they can't make the distance within 3 months... then you wont be attacked until the following year (so you end up with a year break in between).
Besides scouts, information of your site can be passed by visitors, and that information can be collected by enemy civs, at more neutral locations.... this process of trickled information takes more time than it should, and is passing on old data about your site, not current data.... welcome to 43.xx