Learning how DADA2 works
If you haven't already, please spend some time with the DADA2 preprint, this walkthrough of DADA2 for ITS by DADA2's creator, and the ITS tutorial I linked above. There are also many in-depth discussions of how to choose DADA2 parameters on this forum, as well as many great discussions about fungal ITS workflows. The 
search tool is your friend.
Your approach with ITS data may be very different from your approach with 16s (e.g. you may not want to truncate with ITS). Understanding the tools will help you make good choices in both contexts.
Sequence Lengths
@cherman2 hints at this in your other topic:
Next steps:
Once you have taken some time to learn about DADA2, and have developed more specific questions, please feel free to to post them here.
Here are two questions that may help guide your exploration:
- Why do you think your results are bad? (Not why could they be bad, but what evidence makes you think they are bad.)
- Why do you suspect DADA2 is at fault?
It takes many steps to produce a taxonomic barplot. Your data has likely been preprocessed in some way, imported, trimmed, denoised, classified using a classifier built on some kind of database, etc. If your results are not what you expected, you will have to consider which of the many steps in the process might be at fault.