The main reason behind a Workflow no longer processing forms, is usually because there has been a change made to a form or to the Workflow itself, which has disrupted the ability for the form to move on to the next Stage.
In order to find out which forms have not been processed through the Workflow stages, in the Web Portal go to Workflows > Workflows List > select View for the Workflow attached to the forms which aren't processing. In the view section, you can select each stage of the Workflow in order to find which forms are stuck in what Stages.
If you select Edit for a form, it will take you directly to that form in the Web Portal, where you can potentially edit a field if that is the issue.
To find out the conditions attached to the Workflow stages, so that you can compare it to the Form(s) that aren't processing, go to WorkFlows > Workflows List > select Edit for the Workflow in question.
If you go to Step 2, then you can go through each stage and the conditions that must be met for the form to move to the next stage. Then compare it to the form(s) that haven't moved on, and make and save any adjustments that need to be made (to the stage or the form) before submitting the form again.
