This is probably due to an insufficient number of kits available at the supplier depot and can have several causes :
If you did not set a frozen period, the demand at the supplied depot(s) and sites can lower your stock before your planned shipment is supposed to be sent. For example, a shipment could be triggered by the system, before your planned shipment, and reduce the number of kits available in the lot you want to use.
If you set a frozen period (and therefore freeze automatic shipments to depots triggered by the system) and that the central depot is supplying both local depot(s) and sites: If there is a demand at sites and the departure date is in the future (or the same day as the simulation start date), the sites will be supplied first and thus this demand could lower your stock and impact your planned shipment.