After you file

i assume you mean 1099-R.  it should have shown something in box 5 and the taxable amount shown in box 2a.   if the showing the full amount is taxable (box 2a) you need to contact to find out why.  without a corrected 1099, the IRS will go by what they have and reject the amended return.  if you misentered the data go ahead and amend.