The message is a result of the ATP check. To see the availability situation on a system, then you need to use transaction CO09. In your example, you will need to access CO09 with material = 35.31.28 35.31.28, Plant = (whatever plant material exists in), Checking Rule = "B" (Unless you are using a special stock scenario).
In the far right column of CO09, you should see if there is stock available for this material. SAP note 89362 and article 1513607 give an overview of how to read CO09.
At delivery level, the ATP check is turned off / on at item category level. Therefore, check if the ame item category is being determined for non FOC items. If not, compare the item categories in 0VLP and check if the ATP check is turned on for all of these item categories (I suspect that there may be item categories being used that have ATP turned off while the item category being used for your FOC order has ATP turned on).
If none of the above helps, then you should post a screenshot of your CO09 screen here and we can see exactly what the availability situation is on your system.