visualforce - salesforce apex refresh VF pageblock section -

I am trying to refresh the page block section based on the value of a selected list. Here is VF:

  & lt; Top: Page Block Features & gt; & Lt; Apex: selectList size = "1" value = "{! ReasonCode}" & gt; & Lt; Top: Select option value = "{! ReasonCodes}" /> & Lt; Top: Action Support Event = "onchange" rerender = "a" /> & Lt; Apex: Action Support Event = "Incomplete" Action = "{! Disapproved Reasoncode}" render = "Order" /> & Lt; / Supreme: selectList & gt; & Lt; / Supreme: pageblockSectionItem & gt; & Lt; / Supreme: pageBlockSection & gt; & Lt; Top: Page BlockSign ID = "Order" sung = "{! IsAcceptedRC == true}" & gt; & Lt; Highest: Output Label Value = "Order Number" = "odNum" /> & Lt; Apex: inputText id = "odNum" value = "{orderNumber}" /> & Lt; / Supreme: pageBlockSection & gt;   
 I have made a bunch of different events, but no one does the work. Here, the accepted resonance function:  
  public page reference is accepted Recycle () (if (reasoncode == 'accepted offer') {isAcceptedRC = true;} other {unacceptable RC = false; } Return Null;}   
 It seems very straightforward but it does not seem to work ... Surely I change the selection list to = 'accepted proposal'   
 
 
 Add Apex: outputPanel above Apex: page when changing selection options Block section and render output panel.  
 Sample code:  
   gt; & lt; apex: selectList size = "1" value = "{ ! ReasonCode} "& gt; Top: Select Option item label =" test "item value =" test "& gt; & lt; / apex: selectionoption & gt; & lt; head: selectoption item label =" Accepted offer "Item Value =" Accepted offer "& gt; & lt; / apex: selectOption & gt; & lt; Apex: Action Support Event =" Enchen "Action =" {! Iac Accepted Re-Syndication} "render =" Estpanel "/ & gt; & Lt; / Supreme: selectList & gt; & Lt; / Apex: pageblockSectionItem & gt; & Lt; / Supreme: pageBlockSection & gt; & Lt; Apex: outputPanel id = "testpanel" & gt; & Lt; Apex: pageBlockSection id = "Order" sang = "{! IsAcceptedRC}" & gt; & Lt; Highest: Output Label Value = "Order Number" = "odNum" /> & Lt; Apex: inputText id = "odNum" value = "{orderNumber}" /> & Lt; / Supreme: pageBlockSection & gt; & Lt; / Supreme: outputPanel & gt; & Lt; / Apex: Page Block & gt;