Help! 3 year old daughter is holding pee

My daughter just turned 3 in March, and potty training is not going that well. We starting putting her in underwear and the only place she will actually pee is with her nanny at the program they go to daily. At home she will hold her pee and refuse to use the potty and cry and run away if we put her on it. Then she will eventually have an accident. We've tried rewards but she's not interested. I'm actually concerned about the pee holding as well as it doesn't seem healthy-this morning she held in urine for about 5-6 hours! Any tips? I know about the method of taking away underwear and having them in the nude, but my husband is a bit reluctant to try this just yet. Is it possible she's just not ready?