What determines when a custom item checkbox field is available to a workflow set field value action?

Hi Everyone,

I am creating a workflow that will clear a custom item check box, but the desired field is not available for selection in the workflow's set field value action parameters. In looking at the available fields, I see that 1 of 3 custom item check box fields is available, just not the one I want to use. I do not see any significant difference between the 3 fields and can not figure out why only 1 of the 3 custom item fields is available to the workflow.

Thanks for any insight.

John