ok .... your problem is that you go to failure at each set ...when u go to failure at each set u'll have to decrease the weight or reps every set..... because your muscles are fatigued after going to failure....if you want to be able to do 15 reps every set lighten the weight at the first set and don't go to failure , then you'll be able to even increase the weight every set withought decreasing reps ..... note: going to failure is not essential for size and mass .... but u should go to failure from time to time , some ppl go to failure at their last set ...
another way to alternate between reps is pyramiding ..by increasing weight at every set while decreasing reps ex: 12,10,8,6 while increasing the weight...or 6,8,10,12 while DECREASING weight lifted (not for hypertrophy (size))
but alternating every week is a good method to mix strength with size and endurance....
FACTORS EFFECTING YOUR ABILITY TO LIFT A WEIGHT = REST INTERVAL , NUMBER OF REPS , GOING TO FAILURE , THE AMOUNT OF WEIGHT LIFTED , TEMPO, RANGE OF MOTION ....