A good trainer will notice when the horse stops having fun. This is not unusual during any training program. The horse may lose its sparkle, even get a little depressed. Because most people think that a horse that walks around with his head down, appearing clam, is really doing right. But that isn't necessarily so if there is no spark.
Spark is what makes winning horses.
Don't get greedy and force your horse on the days when he loses his spark or seems a little bit depressed. There's no good reason to push. If you do, he'll be doubly disinterested or depressed tomorrow. When your horse loses interest in the program, you have to back off your training schedule and help him find something to be interested in again. I'm not saying that you should stop working a horse every time everything isn't going right.