your follicle size was perfect, but dear this is not the only factor for
getting pregnant. the sperm should fertilize the egg, may be it didn't happen because the intercourse was not timed correctly, may be the sperm did not have enough energy, or just did not happen. on the other hand, may be the egg got fertilized but didn't really implant, so no pregnancy. see, there are many reasons, but also, this does not mean you can get pregnant, these follicles give you a higher chance.