"Unprocessable Entity" is not anywhere in WeBWorK code, so this must be something Canvas is reporting.
Usually with LTI 1.3, when grades pass back for some users, but not all users, it means that WeBWorK has not yet learned the "lis_source_did" for those users (the ones whose grades are not being sent). This should only happen if those users have never actually followed an LTI link from Canvas into WeBWorK. Are your students able to log in with a password instead of using an LTI link from Canvas? If so, can you ask/require that they use Canvas? If you want to require it, there is a setting in the Course Config page, LTI tab, that will require all users (including yourself) to enter WeBWorK via Canvas.
If you have access to the database, you can directly check if users have their lis_source_did in the database. You would use something like this:
select user_id, lis_source_did from `courseName_user`;
where "courseName" should be your course's name. If you see NULL entries, that would confirm that those users haven't used an LTI link yet. But maybe "Unprocessable Entity" means those lis_source_did are there, and something is wrong with them. If you see them there, you could look to see if they seem corrupted.