Why cant I unlock my phone using the power button?

when i try to unlock the phone by using the power button or the home button the display wont come up , only the backlight will!i have to press the home button or the power button for like 5-10 times to get my display up! most confusing part is that the power button and the home button both works fine when the phone is unlocked ! this problem only occur on the lock screen!please help me :(