ModalPopup of Masterpage not showing

HI,

I have a situation where I have a ModalPopupExtender in 

1. Masterpage

2. Contentpage say Search

Modalpopextender of Masterpage stops working if I have opened contentpage where it having another Modalpops. But if a content page is not having any ModalPopup its working fine. Could someone aware me with these common issues. 

FYI, I am calling ModalPopExtender1.Show to open the popup.

Please assist.

Thanks & Regards

Jay