By the time I get home it's typically a motivation problem. Also, trying to find a gym that is affordable yet worth going to. I hate going to gyms that are filled with people and you have to wait for everyone to get off the machines.
That's definitely the external factors. As for physical factors, I'm a skinnier guy and it's typically tougher for me to build a muscle mass. I have to increase my calorie intake by quite a lot and I also have to make sure to eat on a regular schedule. I typically go 3 months of heavy lifting with a managed diet, and then I go 3 months of just regular lightweight lifting and not as many calories and work on cardio.