What Causes Persistent Vomiting Despite Taking Zofran?

I was prescribed Zofran for vomiting occuring due to flu, after being given 3 liters of IV fluids and potassium pill at the ER early this morning. I have had the flu for 5 days, started taking Zofran this morning and have taken 2 doses. It seemed to be working this morning, but in the last few hours i have vomitted twice and am still a couple hours away from my next scheduled dose. I am just wondering if this is OK. I have never had