I'm not sure what you mean, but if it is dynamically created controls, then you need to make sure to re-create the button on postback. Otherwise the event will not get fired!
public partial class AssetMasterDetails : System.Web.UI.Page
protected void Page_Load(object sender, EventArgs e)
Control uc = LoadControl("AMSAssetAdd.ascx");
There are code errors in your posting f.x. you declare PlaceHolder1 which is not possible in the scope in LoadControl, it's already declared, you add the uc twice to the placeholder, event name in code behind not same as event name in ascx and more :-)