It seems that details come out as they are applied to the argument made at the time.
Adjuncts are not a monolithic group. As someone in a position to observe many types of adjuncts, I point out various, equally true things during an ongoing discussion.
Yes, we've had adjuncts who decide they'd rather have a full-time job somewhere else than the part-time job we can offer. We wish those adjuncts well, even when the resignation comes during the term and we must replace them midstream.
Yes, we've had adjuncts who have been offered full-time faculty jobs turn us down and keep their part-time jobs with us because they prefer to be part-time doing the parts of the jobs they like instead of having to do the full faculty job, even with more pay. We have then hired other people who wanted to be full-time faculty doing the job we have to offer.
Yes, we've had adjuncts who teach for free (donated their salary back to the college) because they just love to teach.
Yes, we have adjuncts who are professional fellows for us who teach term after term as either retired professionals who want to teach a course or two or current professionals who are happy to teach as part of their income. Generally, these folks have special contracts well above the base pay because their expertise is hard for us to acquire otherwise. Some of those folks are active participants in their departments (attending meetings and such) while others are happy to come to social gatherings and be updated in writing about changes in the curriculum/assessment that affect how they teach. It depends on the contract they negotiated and whether they want the full faculty experience or mostly the interacting-with-students part.
Yes, we have non-tenure-track full-time folks who have negotiated contracts that compensate them for service as well as teaching.
Yes, we have people who have multiple advertised-originally-as-part-time jobs with us to come up to a full-time benefitted position that includes teaching, but does not have full-time faculty requirements.
Yes, we have people who were hired into a full-time staff or administrator position who also teach a class or two as an adjunct for extra money.
Yes, we even have a few fly-ins who were hired to teach something we couldn't otherwise cover who are working for the base pay. Those are the people who tend to leave us midterm if something better comes up. They are a tiny minority of the people who are working for us in the category of "adjunct".
I have similar stories about full-time faculty: some of whom sweat blood for the school, some of whom are clearly doing the absolute minimum to not get fired on the spot, and some of whom are in the middle. When we only have about 60 instructors of any kind at the school for any given term, knowing everyone's story is much easier.
If you are somehow conveying to part time adjunct faculty that, in your view, they aren't doing enough work for the college, when they are in fact doing every bit of the piecework identified in the contract, then that would explain why some are refusing to work for you.