Ok, that almost certainly means you can't hear the difference between the aspirated and unaspirated allophones. That's absolutely typical of native speakers: they can't hear the difference between allophones.
Now try standing in front of a mirror, and hold a Kleenex (tissue...) in front of your mouth while pronouncing those words. You'll probably see a tiny flutter when pronouncing 'spill', 'spy' etc., but a large movement when pronouncing 'pill', 'pie' etc. The tissue is showing a difference that your ears have a hard time with. Not because the difference is hard to hear--a Thai speaker would have no problem--but because the sounds are allophonic in English (but phonemic in Thai).
Now try standing in front of a mirror, and hold a Kleenex (tissue...) in front of your mouth while pronouncing those words. You'll probably see a tiny flutter when pronouncing 'spill', 'spy' etc., but a large movement when pronouncing 'pill', 'pie' etc. The tissue is showing a difference that your ears have a hard time with. Not because the difference is hard to hear--a Thai speaker would have no problem--but because the sounds are allophonic in English (but phonemic in Thai).