Is it ever the case that the doctor is unable to reattach a detached retina?

Yes: Soemtimes, if you have a severe traumatic injury, the entire retina may not be able to be saved. More often, the retina can be repaired anatomically, but the patient does not see well. See your eye doctor for more info.

It is uncommon: In a small percentage of cases, such as long-standing retinal detachments, excessive scarring, or in eyes that have had multiple surgeries, it may not be possible to reattach a detached retina. In most cases, the surgery is successful.

Sometimes: Most retinal detachments, unless neglected or unrecognized, can be re-attached. If the central portion, the macula, comes off, then the vision recovery is much less certain even if the reattachment is successful. But a few reach the point where the surgery cannot help.