why isnt anything progressing anymore in building muscle?

im 16 years old born in 1994 and at the moment a weigh just abit off 70 kilos. im 171cm tall which is not tall at all, but im not fat either. i work out about 1 hour and a half about 5 times a week. and i want to hit 75 kilos. its been about 15 months of working out of me and i have changed like massivly i use to be skinny and all. But for the last three months ive seen no progression i havent bulked up any more than 3 months ago... although ive hit my dumbell weights higher like for example i do 16 kilos for my biceps 7-10 reps for one set. but i havent changed at all. y is this?
 
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