Its genetics, no matter how much in dosage you may take, if you have already dabbled with your hrt dosage, your probably not going to get any more response. My Endo, has me a low dose of estrogen of patches and spiro, I may probably in December increase my own hrt, since I let the Endo call the shots this time, once I get a baseline at my first year mark, I will dabble somewhat with my hrt dosage.
P.S. I'm also sorta flatchested, but I do get weird signs from my male counterparts at work though, I'm 9 months going on 10 months on the 28th of this month. I hear you need to let the hrt do its work for 3-5 years, before considering implants.