This is most often caused when you try transfer more than the transfer limit you have set at your bank.  You can change this in a few clicks at your bank's online site or app.  

How do I up my limit at my bank?

When you make a payment through POLi you will only be able to transfer an amount up to your Daily Limit.  

This means you may need to change your daily limit at your bank (this is easily done online).  For example, with CBA this can be found in "Settings" --> "Daily Transfer Limit".