This works as expected with me. Do you use a custom automultiplechoice.sty? Can you provide a minimal working source file that shows the problem?

Hey, Alexi. First, thanks for your sooner answer. Second, of course I attached the Latex code lines that I think would be useful for this scenario

%%% An example with a separate answer sheet

\documentclass[letter]{article}

\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}
%%usepackage
\usepackage{graphicx}
\usepackage[font=small,labelfont=bf]{caption} % Required for specifying captions to tables and figures

\usepackage[box,completemulti, lang=ES, separateanswersheet]{automultiplechoice}
\begin{document}

\AMCrandomseed{1237893}

\def\AMCformQuestion#1

\setdefaultgroupmode{withoutreplacement}

Also, I attached an AMC PDF file with this Latex code. I hope this helps.
Thanks

Dear Jose,
could you try without the line?
\def\AMCformQuestion#1
I don't understand whats its purpose.
Best regards,
Rafael

Hey Rafael. Me neither. Then I tried without that line and It works !
I appreciate a lot your time to help me.Thanks.

P.S. I attached an image with some sections of the PDF file.