Hi @cintia_martins!
How are you choosing your trim and trunc length parameters? There are lots of good discussions of that here on the forum (incl. 1, 2), as well as in the Atacama tutorial, and in the DADA2 docs.
It's common to lose sequences if DADA2 does not have enough nucleotides to successfully join forward and reverse reads. This can be caused by short raw sequences, but is often caused by narrow trim/trunc values.
Your quality scores for both forward and reverse look strong on the trim-left side, and you might not want to remove that data. At the same time, you might be introducing too many low-quality reads by setting trunc to 260 for your reverse reads. It's not necessary to set forward and reverse to the same values. If more tinkering doesn't yield improved results, let us know!
Chris