Assuming they were deemed negligent, and they are liable for your loss. The next step is determine how much damage you should get. The damage has to be foreseeable.
Your missed opportunity in stock market is not foreseeable damage because nobody can predict the stock market. Your damage is usually market interest for those missed three weeks.
For one's action to be deemed "caused" your damage, they have to meet two conditions:
1. but-for cause. The action has to be in the chain of events that leads to your damage. For example, Starbucks messed up your order so you were delayed, and when you exited Starbuck's parking lot you were hit by a truck. Had starbucks not delayed your order, you would not be hit by that particular truck, so starbuck's negligence is a but-for cause for your damage.
Same is in your case, had they not delayed your money, you would have made a profit.
2. proximate cause. The damage has to be foreseeable by a reasonable person from the negligent action.
In Starbuck's case, It's not reasonable to expect Starbucks to foresee that messing up customer's order would put them in danger of traffic accidents, because it would be equally likely that their messed up order SAVED customer from an earlier would-be traffic accident.
Same here, the bank delayed your payment due to negligence, that's true, but could bank foresee that the delay would cause your missing the opportunity? What if stock rose and you didn't make the money?
So your demand failed because your so-called loss is not foreseeable.
