Enough rest between sets?

[Collegekid]

I usually do 3 or 4 sets per exercise. On my first set I always lift the most weight I can for 8 reps. Bu on the second, third, and fourth set it's harder and harder to get 8 reps with that same weight, and usually I have to decrease to 6.

Does this mean i'm not resting enough between sets?

[Batman]

No, it's perfectly normal.