If the batteries are ok then the most likely fault is with the switch on the tool, some dust may have entered the tool and also the switch housing.
You may like to have a go at fixing a switch fault in the tool the tool yourself with the information below, but this must be entirely at your own risk.
Please only open the tool at your own risk. CEL cannot be held responsible for damages caused by you opening the casing of your power tool.
Attached below is an image of one of the methods used to clean dust from the internal rocker switch. The battery must be removed before opening the tool.
You tool may be slightly different inside but the same method applies.
To clean the switch we would open the casing and clean the internal rocker switch. The seal around this switch should not let dust inside by may be missing or damaged. We would spray some WD40 or similar into the hole beside the contacts in the bottom of the switch (marked with a red arrow on the image above) we would operate the switch a few times to loosen any obstructions then seal the area with grease once it is working. Some additional grease around the edges of the rocker switch can also help keep the switch sealed.