Expand the web app that contains the site collection where this is occurring
Expand “Sites”, then the relevant site, then “Lists” and then expand your list
There should be at least one Event Receiver labelled conditional start but there could be three for a list and four for a library. The “Type” could differ (“ItemUpdating”, “ItemAdded” and “ItemUpdated” plus "ItemCheckedIn" for libraries)
If there are none republish a conditional workflow, this should trigger the receiver creation.
If the Event Receivers are present, or re-creating them as above does not resolve the issue the NintexAutoStartRules.xml file is most likely corrupt anding fail to take on new conditional start data.
Open the List in SharePoint designer and navigate to: All Files > The affected list/library > Forms
Find and Delete NintexAutoStartRules.xml
Re-publish any workflows set to start conditionally, this will re-create the NintexAutoStartRules.xml file.
There are two possible causes:
The event handler being used to trigger the List Workflow is not present.
The NintexAutoStartRules.xml file is corrupt and not indexing the start conditions correctly.