Ive been working out for approx 8 weeks and since almost the start Ive been able to curl a 20 pound dumbbell with ease, but when I move up to a 25 pounder I can only go about 5 or 6 reps in a set until I am completely exhausted. I usually go for 10 reps in a set, 8 as a bare minimum so it is really ticking me off that I can't progress past 20 pounds. I can pump out 3 sets of 10 with a 20 pound dumbbell and not even feel it.

Any advice?