I think what may be causing the problem is that the body used in the combine-subtract no longer exists, this is the error you get anyway trying the mirror without the fillet, so that could be causing the problem.
A work around could be to mirror the body used in the subtract, before you do the subtract. Then use both bodies in the subtract operation and add fillets.
As Kyle Mitchell proposed, mirror the body and after that combine.
Dear Mary Hall,
As I saw the model cavity is removing material more than the cylindrical bore at the bottom and due to that the portion the mirror of that feature will not work. because it needs to remove the more material from out side at the bottom portion.
The features obtained after combined operation will not work for any models, you have to use multiple bodies for multiple locations.
After completing the part upto which you created, mirror the whole body along right plane with unchecked marge box, this will create 2 bodies. After this you can use the combine option for generating common region. this will make similar feature at the opposite side.
Thanks everybody. I've just got back to working on this and have resolved this issue with your help
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