Sounding nasal while singing songs but not during vocal warm ups?

So I warm up my voice before I sing using my keyboard. Doing Oo, Ee, Aa, Ah for my lower range to middle, and from middle to high range I do the same sounds but more open, while breathing from the diaphram, I do a few other little warm ups as well but they're a bit harder to explain... I always check to make sure I'm not nasal and I never am, I record my warm ups to make sure... but then when I actually sing a song... my voice goes really nasal especially with higher notes... low to mid is fine, but high notes go really nasally, I've made sure my tongue is right at the front behind my bottom teeth, I'm making sure my throat is open but I still seem to sound rather nasal... any other techniques I could use to help fix this problem?