some are given 2/2 points and some 0/0.

Can you show us an example?

Your right, Alexis, in fact the example shpuld be:

\begin{questionmultx}{question}\\
Some text ... *\VQfor{}* ?
\parbox[c]{.92\linewidth}{
\AMCnumericChoices{\VQsol}{exact = 2, decimals = 0, digits = 3, sign = false, scoreexact = 2, approx=5, scoreapprox=1}}
\parbox[c]{.04\linewidth}{
\end{questionmultx}}

Excuse my mistake

Here are some partial screenshots of diferents marks to the same question

Analizing questions and answers one by one manually I notice marks 0/2 and 2/2 are all when \VQSol value is zero!
If answered correctly, then they are marked 2/2 and if wrong, 0/2 as expected.
The problem comes with exercises where \VQsol is not zero, as even if markings are correct, then the questions are weigthed null (2/0, or 0/0) and not computed on the exam note

What are the parbox'es for? The problem remains without them?
Can you join a complete source file? I did not succeed including your code without compiling error.

In the parbox there is an image illustrating the question. I try to reproduce the code on a simpler file and upload the code.

Here is the code on a simple file, without images. This is not the real question but gets the same result.

Sorry, please ignore the source.tex on the preceading post. Was not the used code.
Take this new one with this sample answer scan and check that student 103 is given a full score when his answer is wrong

Thanks. From your code,

\FPeval\VQfor{trunc(10+random*10,0)}
\FPeval\VQsol{round((10-\VQfor)*5,0)}

it appears that \VQsol is nonpositive. When negative, it can't be coded because sign=false, leading to some problems.
Maybe a mistake in the formula for \VQsol?

The answer should be positive. But your right as problems come when the result is zero.
Do you think that if the result is positive and nonzero it will be scored correctly?

The answer should be positive

Maybe the correct formula is

\FPeval\VQsol{round((\VQfor-10)*5,0)}

Do you think that if the result is positive and nonzero it will be scored correctly?

Yes.

The point was the result was sometimes negative. This solved the problem.