Make sure you are clear and consistent with your verbal cues or hand signals. Did you say, "Come" or "Come here"? For a "Stand" cue, is your palm facing your dog or away from him?
Say It Once
Repeating cues is the fastest way to dog training frustration. If you want reliability (who doesn't?) you need to make sure you say the cue one time and then help your dog get it right. If you find that you always feel like you have to repeat cues, this is an indication that your dog needs more help on that aspect of training. If this happens, practice 5-10 repetitions where you say the cue, and then help your dog do the behavior by using a food lure or other "helper" such as gently pulling the leash for "come". You need to condition the behavior and associate with the cue by doing lots and lots of repeitions. But, saying it once is crucial for reliability.
When you are communicating with your dog to "Sit" do you bend at the waist a little bit? This is a common occurrence with new trainers. If you add extraneous body movements, your dog might think that this is part of the cue. Your dog might be confused (and you might get frustrated) if you ask for a "Sit" next time without bending at the waist.
Blocking occurs if you give a hand signal and a verbal cue at the same time. Why is this a problem? If you want your dog to learn both hand and verbal cues individually, then he might be confused when he sees just one of them if you sometimes put them together. You might have to always give both cues for it to make sense. He might also not learn one of them if he pays attention to the cue that he is more familiar with.
Keep Your Dog Interested
Is your dog engaged in the training session? Is he giving a lot of eye contact. Is he having fun? One difference between novice and professional dog trainers is the skill to keep a dog excited about training. The more excited your dog is the more he will pay attention and will give you a better opportunity for teaching more skills. Use enthusiasm, great treats or toys, move around a lot and reward frequently to keep your dog interested. Also do short training sessions and stop training before your dog wants to stop to keep things fresh.Dog training is a skill that evolves over time. Pay attention to the details of training and you will increase your skills and have a lot more fun.