From your description of the case history it appears that the root cause of hiccups has not been determined. While most cases are due to simple reasons such as gastric distension, there are other more serious causes such as metabolic/endocrine disturbances, CNS disorders, food pipe or gastro-intestinal diseases, drugs just to mention some common ones. Hence the first step is to determine the cause of hiccups.
If there is metabolic abnormality, it needs to be corrected followed by swallowing of dry sugar granules. Sometimes this is adequate. You have given the names of some medicines tried but there is no information on the route of administration whether by injection or oral and the duration of treatment. Instead of prochlorperazine, we prefer chlorpromazine (25-50mg im/iv repeated three times followed by oral administration for up to 10 days). Other drugs found useful are: quinidine, nifedipine, haloperidol and swallowing of lignocaine solution. In the case of neurogenic hiccups, clonazepam, carbamazepine, phenytoin and valproic acid have been found useful.
If every thing else fails, the ultimate treatment is left phrenic nerve block and crush. You should consult a gastroenterologist.