The Lead status option "Not fulfilled" means that there are less confirmed matches than the limit that was set up for the leads. An example is if you send the lead to 100 people and only 1 person accepts it. If you have the setting of max_matches to 4, then the lead will show as unfulfilled because it needs to be matched to more people to maximize its value.
This setting is found under Settings -> General Settings, in the Configuration tab as see in the image below:
The default value is 4 but you could set this to any integer value depending on your needs and how your lead and matches work with your members.