Sorry, that's really stressful. I live in an area that doesn't have heartworm, so my dogs aren't on a preventative - we just do a routine dose of dewormer at the vet once or twice a year to flush out anything they might have picked up. So I'm not really up on the current state of affairs. Is it possible there are drug-resistant parasites in your region? Or might he be on another med or supplement that's interfering with the drug absorption? It'd certainly be worth contacting the company about reimbursement.