A common mistake people make when training is OVERtraining. Funny as this may sound, you’re muscles do most of their growing when you are away from the gym (that’s why nutrition is so important btw). When you are lifting weights you need to give yourself at least 48 hours in between working the same muscle and you should not do more than 10-12 working sets for each muscle (that means about 3 exercises per muscle group tops). Other things to consider: good nutrition is always important, you need it to support your new muscle growth & make sure you’re getting enough sleep, lack of sleep promotes overeating and muscle degradation.

How do you know if you’re overtraining: there are a lot of symptoms…fatigued muscles, sleepiness, depression, moodiness and loss of strength. If you have some of these, look at your training of the past 2 weeks and make sure you are not overdoing it.
If you are overtraining take a week or two off, you’ll find it will do you very well. When you get back don’t overdo it, do your thing in the gym and get out (as a rule you shouldn’t spend more than 45 minutes with the weights, results go down after this anyway).