The problems I have had with chain oilers were usually either lack of supply, loss of pump prime, or partly blocked outlet. The lack of supply happens either because the pick-up filter in the oil tank is blocked by crud, or the rubber hose inside the tank between the pickup filter and the pump intake has split and it is sucking air into the pump. The loss of pump prime happens because somebody has used dirty oil (in my case a friend had used old sump oil for chain lube) and scored the oil pump piston and bore so it had difficulty priming itself after it had been run dry. Once it primed, it worked OK despite the damaged pump. The partly blocked outlet happens from wood chips getting into the nozzle that squirts oil onto the chain. Crude cleanout techniques compound the problem by damaging the nozzle, and you end up with oil going everywhere but onto the chain.