I don't fully understand your use case - I'll make an educated guess that you're trying to exclude certain accounts from contributing towards your totals if the net position per month is = zero. There must be a bit more to it though as if the net position per month per account is zero - it won't impact the totals, so why take it out? Anyway, disregarding my requirement knowledge gap - to support this type of logic you'll need to make use of the Aggr statement. It allows you to compute figures at a grain of your choosing, so you can do your sum(Value)>0 check per account per month and sum up the remainder. Something like the following should do the trick:
... or trying to second-guess your actual business logic requirement, minor tweak to ignore any selections in your net zero check:
Hope it helps & happy Qliking!