So, "imaged" or "re-imaged"? If the former, meaning its the first time they are being enrolled into your JSS, I don't see a reason for them not to show up other than, have you checked to see that the Macs show up in inventory in general? Meaning, are they getting properly enrolled? If yes, you should be able to select them when editing an existing Static Computer Group. I assume they are being enrolled since you mentioned that they can be selected when creating a new group, which leads me to the second possibility....
If you "re-imaged" them and they happened to be in that Static Group prior to the re-image, they will land back in that Static Group after being imaged. Static Groups use the hardware information for group membership, and as long as a Mac isn't deleted from inventory, when it gets re-imaged, the JSS picks it back up based on the UUID and it goes right back into whatever Static Groups it was previously in, just with the new computer name and/or software information.
Ok, never mind my post. Seems you figured it out while I was typing up my response!
