【求教】lcd1602 i2c不能自动刷新

1602的屏幕不知道为什么，不能刷新，连初始的hello world都需要到后面的i2c的板子上面调节对比度的旋钮上面旋一下才能出现hello world，就算下载新的程序，1602还是显示上一个程序的字符一直不动。
做dht11和1602连接的完全做不下去了
//dht11 data=pin2
//lcd1602 scl=A5;sda=A4
#include <dht11.h>
dht11 DHT11;
#define DHT11PIN 2
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27,16,2); // set the LCD address to 0x27 for a 16 chars and 2 line display
void setup() {
Serial.begin (9600);
lcd.init(); // initialize the lcd
lcd.init(); // initialize the lcd
lcd.backlight(); //Open the backlight
}
void loop() {
int t=DHT11.read(DHT11PIN);
Serial.print("Humidity (%): ");
Serial.println((float)DHT11.humidity, 2);
Serial.print("Temperature (oC): ");
Serial.println((float)DHT11.temperature, 2);
// put your main code here, to run repeatedly:
lcd.clear();
delay(2000);
lcd.print("T is :");
lcd.print((float)DHT11.temperature, 2);
lcd.print("C");
lcd.setCursor(0,1);
lcd.print("H is :");
lcd.print((float)DHT11.humidity, 2);
lcd.print("%");delay(2000);
}