The low jet probably has a partially blocked jet. It isn't unusual for paraffin or petrol fuelled burners to become blocked and the jets have to be cleaned regularly with a jet cleaner or pricker - a thin hard steel wire clamped in a pressed steel holder. Modern or premium products often have an internal jet cleaning mechanism built in.

It is rare for propane or butane powered appliances to suffer blocked jets but it does happen sometimes. If the jet is accessible it can usually be cleaned with a pricker of the right size but if it isn't accessible or the pricker provides only a temporary cure, the appliance will need partially dismantling and the jet (or nipple) removed for cleaning.