Ever have your dog execute a command perfectly several times in a training session and then act like he'd never heard the word when you gave him the same command later in the day? Chances are, that's the "school's out" syndrome.
In the morning you put the leash on the dog, take him to the training area, do a few exercises, and after a correction or two he works like a clock. The routine tells him that "school is in" and he is going to have to work. But, later in the day, when you use the same command, the formal training session has not been established and naturally he will not understand that he must comply.
The answer is to practice random commands when the dog is not expecting them. To be effective, they must not be done at the time of the regular training session. Give a 'sit' or 'drop' command when your dog is occupied with something else. Always precede the command with the dogs' name, to let him know you are talking to him. Don't wait to see if he obeys the command or not. Get to him as quickly as you can and pop him into the sit. As soon as he's sitting, give him a few words of praise, pat him on the shoulder and release him with an "okay" command. Let him go back to playing or whatever he was doing. The idea is to do nothing that would make him think he was back in school.
Do not give another random command for at least five minutes and don't do any more than half-a-dozen trials in a day. Remember we're trying to get him to obey a single command at a time when he least expects it.
Good luck with your training!
Neil