Coincidentally as you said this, I went from 50-60.
I wanted to ask this question yesterday but didn't have the time, that's when I was stuck ata 50% for what seemed like hours.
I'm just giving it a charge off my notebook, i'm at 90% right now. Check on what your PC/notebook is running in the background, it may have something to do with power, i put my notebook in the sleep mode while charging my phone.