Richy,
I think its good to set a fair amount of working hours say 35-40 if you want to increase cash flow with collecting, let the colelctor start a bit later that day say he work 11-7pm I think a happy team is a good productive team, its NOT all about the money, but working conditions, how you are treated/spoken to, its also about having realistic expectation on both sides.
You could deffo get someone to work the hours you want with the pay you are paying, but how good would their work be? What would the quality be like?
Is collecetion a big part of your business, can you try and convert them to other methods, this would remove the problem completley.
If I had guys working till 7pm, it would not be collecting it would be canvassing

- However if its a must and you cannot/do not want to change your current hours, maybe tell the colelctor that they get a percentage of any moneys colelcted as a bonus.