Why am I not sweating when I work out? I'll be a little damp on my lower back but wont sweat a drop from the neck up. I am drinking plenty of water.

Fit? As you grow more fit your body will change how it sweats; it may be that you need to increase your level of activity as you've become too efficient to break a sweat at what you are doing now. There are rare cases where people are unable to sweat at all, but this is unlikely. If you are are really worried, see a dermatologist.