Uh oh. I checked the game again you are right.
The button for collecting sunlight doesn't reshow.
This is the effect of my late night debugging session.
I was fixing the bug where you could infinitely collect sun after going underground and back.
Now the collect sunlight button doesn't reshow between days.
The solution is to collect water first (or go underground and back) and button should pop up again.
I am terribly sorry for overlooking this bug. I hope it doesn't stop anyone from enjoying our game.