These types of heater have a thermal safety cut-out to prevent overheating. If we make the assumption that this thermal cut-out has not developed a fault, your problem is probably a lack of air flow.
In general, this type of heater needs to be installed at least 600mm (2 feet) down from the ceiling to ensure a good air flow through heater. If yours is fitted closer to the ceiling than this, I'd suggest moving it down to solve the issue.
The next step would involve changing parts if all the intakes and fan blades are dust free, so may be better replacing with a new heater.