Immediately giving a "no good" signal could be confusing. Suppose there is a diving catch by a grounded receiver at the six-inch line. If you signal that the try is no good, the Team A coach is going to flip out because he thinks you're signaling incomplete. According to his wishful thinking, that catch probably happened in the end zone, and you cost him two points by screwing up the catch call.
So I like to indicate the result of the play first. If a runner (including the receiver in the above example) is down within a yard or two of the goal line, I'll sell his shortness by pointing at the ground a few times, like when selling a call of down vs. fumble. That's basically the best way I can think of to sell a no-touchdown call. (On a numbered down, the stop-the-clock signal will sometimes do the job too.)
Then after a beat I might signal the result of the *try* by giving the "no good" signal - but you know what, why not leave that to the R facing the press box?