here are some reasons...
give your muscles 48 hours- if you work out the same body group two days in a row they will actually decrease in size, so you need to do upper one day, and lower the next, or some sort of a alternating schedule to avoid this.
watch your workout diet- your muscles take in about 80 per cent of nutrients after a workout; up to 45 minutes after. after this point they basically don't receive anything, so it is very important to have a fruit juice or something high in sugar to replenish your muscles after a workout. it doesnt have to be after, it can be during or before even, just make sure that you have the fuel for your muscles to use. also, protein is important to have for a workout, so you should make a protein shake with fruits for the best thing to drink after you work out.
plateau- this happens when you do the same exercises and weight for 2-4 weeks, try to switch it up and lift more even if you're not quite ready. (you can do reverse sets, or try similar exercises for the same muscle group
sets and reps- you should also vary these, go for endurance one week and lift 15 reps then go for strength the next and go for 8 reps. also, you can do a circuit where you do like 4 exercises on after the other for about 2-3 circuits. also, you can try 'pyramids' which you do 5 sets starting with low reps to high to low, or reversed (high to low to high)