I was not with you in confession

So I cannot directly comment too much there (and if I was I could not!). But can there be cases where it becomes not possible for the Penitent to confess there and then all the mortal sins - say due to not being able to speak due to tears? Yes cases of such impossibility could arise. If such arise - then in the next confession one would confess the mortal sins that were not confessed due to it not being possible…it if becomes impossible …well then it becomes impossible. But once that “impossibility” ceases the
mortal sins are to be confessed (in the next confession… no need to run there).
The other Priest may not have understood that you had not in fact confessed certain actual mortal sins (unless perhaps they were venial sins).
Venial sins do not need to be confessed but may.
Now I will note for some readers who are scrupulous - that they can be in a different boat. Their regular confessor can direct them in their particular case.
There can be various cases where the Priest must make a judgement regarding how to proceed with a particular penitent.
And a case may arise where one no longer remembers what to confess! We are not expected to be Mr. Spock.