Sony NEX Time-Lapse App - Why is the exposure locked?

I'm attempting to do sunrise across a set of mountains.

$9 to pay for the app (that should have come with the $1000 camera).

The first 5-10 mins are ok. After that - it's all blown out And that's on the "Sunrise" mode!

First Exposure is - 2" exp, F8, ASA100 - 35mm
NEX6 - SEL35F18

So far from my tests

a) All the different "time lapse modes" lock the exposure on the first shot. 
(including custom) All subsequent images use that exposure setting.

b) Changing the mode dial has no impact

c) You can set exposure manually, but this doesn't really suit a changing light environment.

Have I missed something here?

Thanks
Paul