I lose weight more slowly now than I did at the beginning. A plateau is when you don't lose for several weeks in a row. There are two ways you can push through a plateau - cut your calorie intake further, or increase your exercise. Oh, and of course, you can do both
My advice: don't sweat it unless it happens for a couple weeks in a row. I have had weeks where I barely lost a lb. and then the next week had a nice 2+ lb. loss.