After you file

I submitted my amended for 2019 sometime in May, 2020. They sent it back in December, 2020 since I forgot to sign in one spot. So they should of received it on December 17, 2020 from what the post office receipt showed.. And its now been 17 weeks and the IRS website to check amended refunds is still telling me the same thing. About waiting 3 weeks and then it taking 16 weeks to process.. Does anyone know why its taking so long if everything on my paperwork was fine except for a signature? I already received my State Amended Tax Refund 2 months ago for my 2019 taxes I filed last year (2020).