I bought a eurolux insect killer a month ago, so now the bulb just fused after explaining that sometimes the device do not switch on at all

I bought a eurolux insect killer a month ago, so now the bulb just fused after explaining that sometimes the device do not switch on at all. They tell me its not their fault if the bulb fused, I must just buy a new one. I tell the manager how can a bulb last less than a month because i only use it when its necessary, and it can be an electrical fault like i mentioned earlier at times it does not come on at all. He tells me it's on now, so no refund and i must just buy a new bulb. Really terrible service. I have the device for 4 weeks now.