Hi Dustin,
I hope the issue may be null exception..you should apply isnull function in formula like:
isnull({CBC_ByEquipID.CBC}) - isnull({CBC_ByEquipID_Model.CBC})
OR
use another way:
IF isnull({CBC_ByEquipID.CBC} - {CBC_ByEquipID_Model.CBC}) > 0
THEN
{CBC_ByEquipID.CBC} - {CBC_ByEquipID_Model.CBC}
ELSE
0
Please check refer thread if needed: Displaying Positive/Negative numbers from a formula
Thanks,
DJ