u/AnnualVacation716

So my Hydrangea are only 3 years old. Past two year the leaves are blooming with brown stuff on it and they don't grow big anymore. And when they are done blooming they have brown spots holes etc. They look weak. I recently put some acid thing to see if it fixes anything not sure if it's going to work. Do I need to fertilize them if so which one should I use? I have that shake and bake miracle gro thing should I use that? Plants are grounded in the soil and not a pot.