If they are asking to see it again then it's most likely to try to catch you. Which means you need to slow it down and perform it better. I rarely have people ask me to repeat it since there was nothing to see in the first place.
I second this, in my opinion there are two mainly reasons of why this happens:
*They were amased and they want to see it again, because the trick that you just did just defies every logical explanation they had and they want to see it a second time to either try to get a logical explanation, or to try to catch all the little details they may have missed. This also includes the tipical "could you do this trick for a friend"
or
* They have a logical explanation or they actually have a hint of how you did it, take card to mouth for example, they were highly amased when they saw it ofr the first time, but after the feeling of astonishment goes away, the logical part of them kicks in and goes like " I was watching the deck! I should watch his mouth the whole time to see how the card ends there" so there you have it.
Actually those two reasons are somewht linked toguether, but they have their differences, it's not the trick that's failing, neither the spectators. AS previously stated, there is something wrong with either your presentation or technique.