Plateaus & declining rate of muscle growth

Hey Marc,

I've heard that it becomes harder to put on mass as you get larger and more muscular over time, but to what extent is this true? I'm 21 now and have been lifting/bodybuilding since I was about 17. I'm 5"9.5 and started out around 165lbs lean and got up to 195lbs lean within the first 3 years. In the last year I've only managed to put on about 2-3lbs of solid muscle, but my diet and training hasn't gotten any worse - if anything it's gotten better. Am I simply hitting a natural plateau? Or should I be expecting slower muscle growth like this going forward? 

Thanks for answering my question.