why do lights blow regularly on one circuit & very seldom on another, both supplied f

[harveya]

This is for those with much past experience!!
Could someone shed light on why lights on one light circuit fuse regularly and on other circuits almost never?
Consider this scenario:
A circuit on a 10A breaker, using FT&E 1.5mm^2 feeds a three lever 1w switch, 1st switches ceiling fan with 3 lamps, 2nd an outside floodlight and the 3rd 6 x lamps connected in parallel (60W bulbs) outside but under cover. It's the last mentioned circuit that is under question. These lights are constantly being replaced yet the others, as well as lights in the rest of the installation, hardly ever need to be replaced. All is correct as far as the wiring goes, the supply voltage acceptable at the time of measuring, although I can't say what the situation is like all the time. Can someone suggest a possible cause - other that poor quality lamps. CFL's have been tried as well and they don't last either.