Live bets have a small delay between bet placement and bet acceptance, to stop cheating. The delay is normally around 5-7 seconds, and if the odds change during the delay the bet will not be accepted. To place the bet when this happens, you need to accept the new odds and try again.
In addition, live bets can also be temporarily suspended. Such a bet is marked grey in both the betslip and in the odds button. For example, this might happen if there is a goal in a match, or during an all-important set ball in tennis. Sometimes the bet has even been closed and can no longer be bet on. Such bets are marked as red in the bet slip.