Are you a native English speaker? It's common for native speakers to be pretty deaf to the differences between allophones. Try recording yourself in Audacity saying "Tell the stellar tart to start stewing a tall stall too," then highlight the two "tell" sounds and listen to them repeatedly. Also the two "tart" sounds, the two "too"/"tew" sounds, and the two "tall" sounds. Maybe you'll be able to hear the puff of white noise that's missing in "stellar", "start", "stew", and "stall". That's the aspiration.