Although the tubes in your fly zapper may appear to be working and giving out plenty of light after use of around 8000 hours run time the amount of useful insect attracting UV light coming from your old lamps drops to a level where it will no longer be attracting insects to your bug killer. These Replacement UV blacklight bulbs for bug killer machines fit many 10w/20w fly zappers that use 330mm T8 bulbs such as the Kingavon BB-IK110, Aspectek AC220, Vermatik VMR20. Flymatic IK-GN-20, IK-GC1-20 

Specifications:

• Wattage: 10w
• Bulb length: 33cm / 13 inches
• Weight: 60g
• Average life: 5000 hours
• Cap type:  G13
• Quantity: 4 tubes
• Type: T8

It is also advisable to replace the starters in your insect killer when replacing the bulbs as your old starters may not be able to provide enough power to reliably light your new lamps. You can get suitable starters from this listing https://www.ebay.co.uk/itm/293129733044