Problem with E-prime....HELP..

HI,
Is there someone kind to help me..
I'am doing a esperiment with E-prime.
it's a esperiment in sound discrimination.every stimulus is a sequence of three sounds.the first and the second are standard,with the duration of 0.35s.the seconde is different from the others ,but with the same duration.there'are 2 intervals of 0.1s to connect.
we've asked the sobjects to answer as soon as possible when they can find the difference,but some risponse are lost,if the response very fast(at the beging of the second sound).it's very strange because there are more people with all registration.
thank you very much..