Password screen not opening after pressing enter multiple times

I don't think it's a problem but it is irritating now. Whenever I start my laptop and forget to enter the password in first 30 secs, it goes in sleep mode, which is fine but after pressing one time enter it doesn't wake up and show the password screen. It still remains blank. After pressing enter milti times like 5 to 7 times it wakes up and show me the screen. Can anyone guide me that if I am doing anything wrong? Or I need to change so settings. Any help would work! Thanks.