This tool is a cBot and a feature of cTrader is that you cannot change the timeframe while a cBot is running, if you have a very small risk or your stop loss is very small, it could be at the minimum already and the % risk will not change any further, without seeing your settings its difficult to help.
You can find out more about how to use this tool and its limitations in the user guide,
You may also be interested in the professional version of the Risk & Reward Tool that works as an indicator so you can change the timeframe while it's running.