Below is the code and couldn't figure out what exactly is the reason, Any Suggestions??

VF code

<apex:page controller="JournalPage1" docType="html-5.0">
 <apex:form >
<apex:pageBlock title="Journal Information">
<apex:pageBlockButtons location="Bottom" >
<apex:OutputPanel >
<apex:commandButton value="|<-First" action="{!firstPage}" />
<apex:commandButton value="<-Previous" action="{!previousPage}"/>
<apex:commandButton value="Next ->" action="{!nextPage}"/>
<apex:commandButton value="Last ->|" action="{!lastPage}"/>
<apex:commandbutton value="Show Details" action="{!Showdetails}"/>
</apex:OutputPanel>
</apex:pageBlockButtons>
<apex:pageBlockSection >

From Date: <apex:input type="date" value="{!startDate}" required="true" />
 To Date: <apex:input type="date" value="{!endDate}" required="true" />
 <apex:commandButton value="Show" action="{!Show}"/> <br/><br/>
  
 
 <apex:outputPanel id="dt">
 Total No of Papers:<apex:outputText value="{!count}"></apex:outputText>
 <apex:actionPoller action="{!updated}" interval="300" reRender="dt"/>
 </apex:outputPanel>
</apex:pageBlockSection>
<apex:pageBlockTable value="{!Wraplist}" var="rp" rendered="{!(Research.size!=0)}" >

<apex:column >
<apex:facet name="header"> Select CheckBox</apex:facet>
 <apex:inputCheckbox value="{!rp.flagWrap}" />
 </apex:column>

<apex:column headerValue="Paper Id">
 <apex:outputField value="{!rp.rcWrap.id}"/>
 </apex:column>
 
 <apex:column headerValue="Paper Title">
 <apex:outputField value="{!rp.rcWrap.Paper_Title__c}"/>
 </apex:column>
 
 <apex:column headerValue="Primary Author">
 <apex:outputField value="{!rp.rcWrap.Name_of_Primary_Author__c}"/>
 </apex:column>

<apex:column headerValue="Status">
 <apex:outputField value="{!rp.rcWrap.Status__c}"/>
 </apex:column>
</apex:pageBlockTable>
</apex:pageBlock>
<apex:pageBlock id="Table2" rendered="{!Showdetailsflag}" >
<apex:pageblockSection title="List of Authors">
<apex:pageBlockTable value="{!AuthList}" var="a">
<apex:column value="{!a.Research_Paper__r.Name}"/>
<apex:column value="{!a.Research_Paper__r.Paper_Title__c}"/>
<apex:column value="{!a.Author__r.Name}"/>
<apex:column value="{!a.Research_Paper__r.Status__c}"/>
<apex:column value="{!a.Research_Paper__r.Submission_Date__c}"/>
</apex:pageBlockTable>
</apex:pageblockSection>
</apex:pageBlock>
</apex:form>
</apex:page>

Apex Controller

public with sharing class JournalPage1 {

    public Date endDate { get; set; }
    
    //public List<Research_Paper__C> Research{get;set;}

    public Date startDate { get; set; }
    
    public List<Author_Research_Paper__c> AuthList { get; set; }

    public boolean Showdetailsflag { get; set; }

    public List<wrapJournalPage1> WrapList { get; set; }

   

    public PageReference lastPage() {
        return null;
    }


    public PageReference nextPage() {
        return null;
    }


    public PageReference previousPage() {
        return null;
    }


    public PageReference firstPage() {
        return null;
    }


    public void Showdetails() {
         List<Id> rpID = new List<Id>();
         for(wrapJournalPage1 wp : wrapList)
         {
         if(wp.flagWrap==true)
         {
         rpID.add(wp.rcWrap.Id);
         }
         }
         for(Id i : rpID)
         {
     AuthList=[Select Id, Research_Paper__r.Name, Research_Paper__r.Paper_Title__c, Author__r.Name,
                      Research_Paper__r.Status__c, Research_Paper__r.Submission_Date__c
                       From Author_Research_Paper__c
                       Where Research_Paper__c =: i];
         }
         
         Showdetailsflag=true;
    }


   // public Boolean selectionflag { get; set; }

    public void updated() {
        Research=[Select id from Research_Paper__c];
                                
    count=Research.size();
    }


    public Integer count { get; set; }

    public List<Research_Paper__c> Research { get; set; }
    
    public JournalPage1()
{
Research=new list<Research_Paper__c>();

}

    public PageReference Show() {
    
     Research=[Select Id,Paper_Title__c,Name_of_Primary_Author__c,Status__c,Publication_Date__c from Research_Paper__c where Publication_Date__c>=:startDate AND
                                         Publication_Date__c<=:endDate];
                          //  count=Research.size();
                            for(Research_Paper__c rc:Research)
                            {
                            wrapJournalPage1 obj=new wrapJournalPage1(rc);
                            Wraplist.add(obj);
                            }
    
    
        return null;
    }


    
    
    Public class wrapJournalPage1{
    
    Public Research_Paper__c rcWrap{get; set;}
    public boolean flagWrap{get;set;}
    public wrapJournalPage1(Research_Paper__c rc){
    this.rcWrap=rc;
    this.flagWrap=false;
    }
    }
}