Hi,
I have just taken over a customer from another windie. I turned up yesterday to clean and she said I had damaged the lead on the downstairs window. There is a horizontal piece about 9 inches long drooping down. Any ideas on how to reattach the offending lead? It's thin and pretty fragile, not a great start to the relationship really! 
It's time to end the relationship now.
You will find that if it's happened to one strip, there will be many more following.
It's the glue that has failed and when your brush or cloth has moved and stretched it, it aint going back.
Glaziers sell rolls of this self adesive lead but until it's weathered, it stands out as its so bright.
If you go down the replacing route, make sure all the old adhesive is removed, tell them it will weather and if you want to keep the job, ensure they do not hold you responsible for any more that will come off.
Tony