Trouble with variables in Million Pound Drop style interaction

Hi all.

I have been trying to create an interaction where learners start with a pot of money, and they allocate an amount to any of four answers. They could put all money on one answer or distribute it across all four. 

They allocate the money by pressing + and - buttons which displays the amount against each answer. So triggers and variables have been set up to do this. 

I also want the their total pot of money to display and as they allocate money, I want the total to adjust accordingly. 

However, I just cannot get it to work! Most triggers are working, but for some reason, if one of the values gets to zero but there is money allocated elsewhere, the total continues to go up. I've tried all sorts of variations of the triggers. It all worked fine with just one answer option, but now I have two (and will have four in total) I simply cannot get it to work. 

Can anyone help or shed any light? I have posted the story file here in case. 

Thanks in anticipation of your help.