Help with stepchild

Hello!

I am new to all this but I'm literally out of ideas. I have a 3 year old daughter, and my partner and I have been together for 7 years. I also have a 14 year old stepdaughter. Up until about a year ago I thought we were very lucky and had a happy home life. My stepdaughter split her time between us and her mum. I had a very good relationship with her, and she bonded very well with her half sister. But in the last year everything has changed. She point blank refuses to come to our house. She Says she feels left out, and she says horrible things about me. When I've tried to leave her and her dad to have alone time together she says its Because I don't want her around, she won't spend time alone with me (and we used to; shopping trips etc) she's dismissive of her sister even though she is adored by her. My partner still sees her alone (picks her up from school and takes her for tea) he tells me it's nothing personal it's just her age and she just wants to be with her friends. However she's happy to sleepover at her grandparents and aunts house. I've done as much as I can for her over the years, and looking back I can't honestly think of anything I would have done differently. I just feel hurt and confused and I don't know what to anymore. I message her frequently even though I know they will be ignored. I don't know where to go from here.