litter box trouble

Please help. My 2 4-month old cats who were totally litter box trained have developed diarreah and oftentimes go outside their litter box. I consulted a vet and took a stool sample to him which tested positive for roundworm. They've completed their round of medication but the problem persists. Any advice would be appreciated. Thank you.

I would call the vet again and let them know what is going on.