Submitting ACH debits is indeed a gamble. However, a 20% return rate is extraordinarily high in my experience. Even 10% is something to be worried about.
Among other things, see if your ODFI offers redistribution, a feature in which NSFs are automatically converted back into customer debits and sent out again for a second or third presentment. (The debit can't be sent to the customer more than three times in all.) This could help increase your collection rate. Also, especially if these are consumer debits, consider making them effective on dates that coincide with most payroll cycles. Of course, this would have to covered in your ACH agreements with the receivers if these are recurring items, because if your customers have specified a particular date, you can't just change it.