Reason #3: You are inconsistent
Dogs operate in black and white as nature intended, they do not understand gray areas. Your dog spends every interaction with you learning - be sure they are learning what you want. These animals have evolved watching our every move and learning how to live with us.
Consistency in dog training is important with both reinforcement and negative consequences. All organisms seek pleasure and seek to avoid discomfort, save for maybe a few weirdos but that is a totally different conversation.
People get so frustrated and upset with their dogs when much of the time they are inconsistent and reinforcing the very behavior they are upset about. One of may favorite examples would be dogs that jump on people. I can guarantee when this is going on the dog has a heavy reinforcement history of being rewarded for jumping, usually by attention and petting.
If sometimes you correct the dog for jumping but other times you pet the dog whilst jumping on you, you are being incredibly inconsistent and the dog will never stop jumping. When you are inconsistent with your dog, you are being unfair and setting them up to fail. If you’re petting your dog for jumping up on you, you are basically signing a check that pays for jumping. The dog finds this reinforcing and will continue to do it.
When working on a “stay”, you need to let your dog know they messed up the moment the get up rather than when they have walked “x” amount of feet to reach you. Also, you take the dog back to the exact spot they were in, do not let them gain ground. This may seem lit nit picking but it is really just good training.
Another example would be competition “heeling”. If half the time I allow the dog to go a few paces out of alignment and then correct the dog but the other half I correct immediately, the dog is getting very mixed signals. You need to let the dog know immediately whether they are gotten the answer right or wrong. This helps to provide clarity and makes you a consistent handler.
Dogs understand their world in terms of “what works versus what doesn’t work”, “safe versus unsafe”. To better understand this, sit back and watch dogs interact with one another. They are clear and concise with their communication, there is no guesswork. When working with your dog, always consider what lesson the dog is taking away from the interaction.