What are the cases when at the beginning of a training course a dog may cease to obey his master’s commands?

1) the conditioned reflex can be slowed down if it has not been reinforced for a long time by dainty, i.e., by an unconditioned stimulus;
2) the dog during the command could be distracted by a stronger extraneous stimulus

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.