From the IFAB website:
If the referee plays advantage following a yellow-card offence, the card must be shown when the game next stops. However, if the offence was stopping or interfering with a promising attack, no card is shown, as the advantage allowed the promising attack to continue.
It's a crap rule as it enables the refs to be lenient [towards certain teams]. It shouldn't be down to refs to interpret if an attack is promising.