Hi Im 35 Yrs Old And Have 22 Frozen Embryos

hi im 35 yrs old and have 22 frozen embryos . out of which 12 is of grade a quality and grade aab quality, All embyroes seem to be good as per my report and i had an frozen et few days ago but my endo ws only 7mm and today i got my test as negative, Can u pls tell me why my implntation had failed and what chances to improve my thickness n to success my implantatio. Thank you doc