Is there something with my friend?

okay so everytime i invite my friend over to sleepover, she comes up with an excuse to not stay and she thinks its not obvious. For example, one time after a party, she said she would sleepover and we all made big plans around that, but she told us that her mom wanted her home. She does this everytime. I have known her for 3 years and she has never slept over, yet i hav invited her probably 7 or 8 times. Our friendship is getting worse because when i try 2 talk to her about it, she denies it and tries 2 tell me that her mom is just strict, even though her mom isnt strict atl all. I have slept over at her house when her parent are home, but she has NEVER slept over at anyone elses house. i no this might be a personnel problem, but i wish she would open up...what can i do?