Well this very thing happened to me, (and im sure many other people) which led to frustration, and wondering what the hell I was doing wrong...well basically after alot of digging the answer was simple....I wasnt giving my body a chance to rest in the long run. Bodybuilders...espcially the pros...will never tell you that they do not look incredibly big and ripped every day of the year....the only way that can happen is if one takes steroids. Basically the human body cannot maintain a large amount of muscle for extended periods of time...and trying to add muscle when you have reached your "limit" will lead to fatigue, frustration, and a host of other bad things.
So how do you avoid this "plateauing" without sacrificing all your hard work....simple....you must cycle your workouts....very simple....after youve been lifting for awhile you have to go in two phases....on season and off season....on season is where your are pushing yourself to the max and trying to add muscle...I usually do this from January-August. Off season is where you're still workout but maybe not as often...yes you will lose some muscle...but not all...think of taking 2 steps forward with your on season and one step back with your off. I usually do this from September to end of December. Basically I give myself some time off, not focus so much on the lifting, and relax. In this way come January im ready to hit the big weights again, and in a few months ill be bigger than I was the previous on season.
Sounds complicated...but im all saying is give yourself a few relaxed months and dont be afraid to lose some muscle....if you don't....you wil hit a plateau of frustration and do way more harm than good.