Lower register in countertenor voice changes as voice warms up - what can one do about this?

I'm working on switching from tenor to countertenor: the problem is that come evening, when the voice is fully warmed-up, I start to lose the ability to sing the lower notes in head voice and, whereas the F, E, D etc above middle C where doable before, they now have to be taken in a blended/mixed head and chest voice. How can this be rectified?