If the WOD calls for a heavy single, increase the weight until you fail on a rep (and it’s not a blatant technique error). After you have failed once, your last successful attempt is your heavy single for the day. If it calls for a 1 RM, if you fail on a lift, you can repeat it up to 3 times. After 3 fails at a weight, you can still move down and try for a lighter weight. For example, if it calls for a 1RM and you successfully lift 80 kg and then fail at 100 kg 3 times, you can go back and try 90 kg.