I found some more info on D3 antagonists in this paper on a failed (or discontinued anyway) schizophrenia drug which targeted D3. It may explain lack of robust enough effects - either not enough receptor occupancy, or a strong genetic component to the response or lack of response.
"The therapeutic efficacy of D3 antagonism in schizophrenia has not yet been confirmed. A minimum receptor occupancy may be required for D3 antagonism to produce antipsychotic effects, as has been shown for the D2 receptor.9, 10 A follow-up study on 50 and 150 mg regimens of ABT-925 revealed <60% receptor occupancy at the D3 receptor,3 which may have limited the clinical efficacy of these doses. Nonetheless, our results coupled with the reported increased D3 receptor binding affinity for dopamine and D3 antagonists resulting from the DRD3 S9G polymorphism,4 suggest that possessing at least one G allele may confer a wider therapeutic window for D3 antagonists. Similarly, subjects with increased dopamine tone resulting from expression of the COMT V158M polymorphism11 may be less likely to respond to D3 receptor antagonists.
Given the size of the data set, we cannot eliminate the possibility that the observed clinical significance of these markers is surrogate for factors which were not analyzed (for example, ethnicity). In addition, the current findings might not apply to patients not represented in the original clinical trial, such as treatment-resistant schizophrenia. Although sample sizes of the S/S and M/M genotype groups were relatively small, and the regimens did not provide sufficient receptor occupancy to assess clinical significance, these results support the notion that pharmacogenetic analyses may reveal sub-populations of schizophrenia patients who could respond favorably to D3 receptor antagonists. However, higher D3 receptor occupancy by a D3 antagonist should be achieved in order to fully understand therapeutic efficacy in schizophrenia and the effects of DRD3 polymorphisms. Thus, further research in this area is warranted."