Autopostback not working

Hi,
actually i created a form in which dopdownlist has an autopostback property whose function is to change the values of other dropdownlist value upon changing.
it was WORKING PERFECTLY , but when i installed a masterpage to my project and to that page/form , the autopostback property is not working it is not changing other list values upon changing??
need help , what should i do?
Thx
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