Combined loops not working in Arduino

I have an arduino code which is doing tasks like this:

while (a=='A')
{
   //do the task A
}

while (a=='B')
{
   //do the task B
}

These loop run correctly when they are run separately. But the problem comes when I try to combine both loops i.e.

void loop ()
{
   while (a=='A')
   {
     //do the task A
   }

   while (a=='B')
   {
     //do the task B
   }
}

Below is the code in full details:

void loop()                  
{
////// admin
Serial.println("A or B");
delay(500);
char a = userinput();
delay(500);
while(a== 'A'){
Serial.println("Type Your Starting ID Number...");
while(1 ){
while (Serial.available()==false);
char e = Serial.read();
if(isdigit(e)==false) break;
startid = (startid*10)+(e - 48);
}
if(startid<=endid){
for(pageid=startid; pageid<=endid; pageid++)
{
Serial.print("Your Biometric ID # is ");
Serial.println(pageid);
delay(2000);
fingerenrollment(pageid);
 delay(2000);
if(pageid == endid){
  Serial.println("......Memory Is Full....");    
  while(1);
 }
}
}
else{
Serial.println("Wrong Entry.......Please Reset Your Device.....");
while(1);
}
} 
////// user
while(a=='B'){
lcd.print("WELCOME TO iPoll");
delay(2000);
lcd.clear();
lcd.print("Place Your Thumb");
delay(2000);
lcd.clear();
tempid = '\0';
Serial.println("Place your Thumb For Authentication");
delay(500);
while(true){
ID = fingerauthentication();
delay(500);
if(tempid != '\0') break;
}
delay(100);
resp = userinput();
delay(100);  
lcd.clear();  
datatrans(ID, resp);
}
}

If you need any more help, I am here. Just comment.