People fail to reach their training goals more often than reach them. There are 1 metric buttload of reasons why. I can only cover 5 here. Mainly because I only want to cover 5, and no more.
1. Poor planning -
I think this is generally the culprit of all failed training programs. I ask people how they will reach X and Y goals or why they are doing X and Y and they don't usually know. I think of the analogy about the guy getting in his sailboat and just drifting out to sea without ever plotting his course. Death awaits. And so does the death of your program if you plan poorly.
Somali pirates will in fact, take over your crappy training plan and kill you.
2. Unrealistic goals -
If you're 300 pounds and want to lose 100 pounds do not plan a 6 week diet for that. If I tied you up in a cave and just brought you water and a slice of bread each day it might happen then (oh hell I just created the next fad diet. The cave diet. Lose all the weight you want in 6 weeks in the comfort of your cave.) but in all seriousness, be realistic about goal setting. If you just deadlifted 500 after 10 years of serious training, you're not going to deadlift 600 in the next 6 weeks, or even 6 months more than likely. Plan on deadlifting 505 next. If you want to gain 10 pounds of mass you're not going to gain 10 pounds of lean mass without gaining around fat at a 1 to 1 ratio. Understand the amount of calories you will have to eat and how hard you will have to work, and for how long, in order to do that. Realistically 10 pounds of lean mass is a difficult chore for anyone besides a novice, in a single year.
This guy will not be Jay Cutlers size in 90 days no matter what he does.
3. Lack of motivation -
This person has the opposite disease of person #2. Motivation doesn't come from a speaker or book or whatever. It comes from within you wanting to change something for yourself. Motivation will wane if it comes from an external source that doesn't have a significant place in your life. If you want to quit smoking, for example, you won't quit until you WANT to quit. And I mean REALLY want it. This is an identifiable trait. I have found that determined people are excited about talking about their goals. They will tell you without you asking. The wanna-be might mention "I really wanna quit smoking" in a passing kind of way. This is generally not a motivated person. If you are honest with yourself you'll know if you are ready or not. Until you are, don't even attempt it. It's better to bob about in a sea of uncertainty and save your energy than to paddle half ass and waste the small bit you do have. When you make a goal you'll know if you are serious or not. Again, be honest with yourself, that will tell you what you need to know.
4. Too much information -
People say you can never know too much. Boy that isn't true. There is more paralysis by analysis regarding training and diet nowadays than ever. Of course the net is to blame for that. No carb, low carb, carb-cycling, south beach, weight watchers, atkins, on and on and on. Then there is a training program for every possible body you could want. From a beach body to looking like the Hulk, all in 60 or 90 days or whatever. After a while someone trying to do honest research and make a solid plan could get information overload. There are some basic truths we know about eating and training. These things are for certain.
If you take in fewer calories than you use in a day you will lose weight. If you are certain you meet this criteria (and I mean FOR CERTAIN) and are not losing weight, talk to your doctor. This is a basic physiological fact. So if that is not working you may have another issue that you may need help with.
If you eat more than you are using you'll gain weight.
Everything else is up for discussion. So any good weight gain/weight loss plan should start at what you're doing at the table. Keep your plan simple and start with calories in vs out and go from there.
5. Life -
Sometimes you just gotta play the game...
Let's face it, life happens. Sometimes you are doing everything right, and life just interferes. But lets be straight about what that means. I'm not talking about the things you are supposed to be doing like going to work, being a good father or mother or husband or wife. That's what you are SUPPOSED to do. I'm not talking about you saying how busy you are and just can't find the time to train or eat right. Or getting nicked up in training or are tired of being sore. For the love of God suck it up. I hate excuses. No, I'm talking about LIFE like your child gets sick and is in the hospital or a death in the family or you have to relocate or lose your job or are in an accident. Things that REALLY impact your life. In this case, pick up where you left off when you can. No one can plan for these things obviously and it's far more important to take care of those things when they come up than worry about training at that time. When my wife had her hip replacement I got to the gym when I lifted when I could and never worried about it. Taking care of her was more important and now she's 110% (literally as she has never had a sound physiological until now) and things are better than ever. Remember in the grand scheme of life to leave behind a legacy bigger and better than lifting weights and being fit. At least kill someone or make a sex tape that people will remember. That will keep you on the map for a lot longer than being in good shape.