We have to remember that Luke can sense clones in the EU, but that is when he is a full fledged Jedi Knight. One could argue that Luke wasnt grown enough in the force to detect the clones in the first trilogy. By the time we get to the EU, most of the cloned stormtroopers are dead as we see in the thrawn trilogy, because Zahn says that you have to be sparing with the stormtroopers (meaning there arent many left). Those troops could be regular humans. A long shot, but it could fit in.