All beginners make significant increases in both size and strength when they start lifting weights. This is because your muscles are not used to the extra pressure your are subjecting them to - so they grow quickly to compensate. As your body gets to know your workout and what sort of power it needs to survive them, it will slow down the growing process until you stop growing altogether. This is called a plateau - a point at which you make little or no gains no matter how hard you try, and is a problem many bodybuilders face. A possible cause could be insufficient sleep (in which case you should increase your sleeping hours), lack of vitamin and mineral levels in the body (try increasing your meal times and including snacks), and over-training - which is covered later. Try to remedy these areas first before you go for more drastic action.
You are more likely to plateau if you use the same exercises for every routine. This is because your body has adapted and lost the stimulation needed to gain muscle. The way to change this to alter the types of exercises you have adopted to use. Vary their position in your training routine and keep an eye on the weights used. In this way it may be possible to constantly stimulate the body and so avoid the plateau altogether.
If you know these areas aren't the problem, there are many alternative solutions you may want to try. One is to stop lifting altogether and let your body recover for enough time to carry on. This may simply mean extending your rest period by a number of days between workouts so you recover better. If this does not work, try taking a number of weeks off. Then after a week or two, increase your weight back up again an try to progress. Another is just to carry on with higher weights until you feel you are making gains, although this may seem very slow to achieve. If you are concerned about loosing muscle due to under-training then this is the way to go, though it takes a few resting weeks before you notice any major strength loss.
A final variation on this is to reduce your weights and the intensity of your workouts so you only stimulate the muscles enough to maintain their size without trying to grow them. After a month or a number of months your body should adapt to this type of low power training and cope accordingly. Now you are ready to apply more weight again. Your body should grow rapidly to compensate for the increase in weight, just like it did when you first started to weight train, except this time you will already have the muscles you built up from the last high intensity sessions. Many advanced weight trainers believe the body can handle 4 or 5 high intensity periods of growth per year, with the remainder of the time either resting or training at a reduced weight. In this way you should be able to maintain growth and work around plateaus', although your growing periods will be much further apart.
