Amisulparide is the only antipsychotic that's been found, when mixed with Clozapine, to consistently successfully treat Clozapine resistant patients. There is a lit-review published where other meds were tried and pretty much each one had it's fair share of successes and failures but no consistent consensus that the other meds worked.
Problem is amisulparide is not available in the US.
One could, however, request the DEA to have it brought in in special cases.
https://en.wikipedia.org/wiki/Amisulpride
Aside from this ECT, augmentation with Depakote or lithium, addition of antipsychotics of highly differing structures to the Clozapine. As mentioned above if Clozapine fails get blood levels of Clozapine and norclozapine. Also consider genetic testing to see if they're a hyper-metabolizer or if the genes just suggest the med won't work.
If these fail consider Reserpine.
Another thing. I had a patient that did respond to antipsychotics but only after several weeks. That said the "late responder" to antipsychotics doesn't hold up in studies.