Dear Stepdad,
My partner and I have been together since my daughter was 2, she is now 4. She and my partner have always had a good relationship until recently, every time her biological dad lets her down, or shows up unexpected she acts out on my partner. Lately she has been telling him she does not like him, yelling at him and shouting you are not my dad. We have always explained to my little girl that my partner is not her dad.. but it is as if her dad is telling her things to resent my partner as she is loving and happy towards him when her dad is not around. –Tash
Tash,
Don’t panic. This is a stage that will pass quickly. As little kids’ brains develop they tend to categories people into groups. For instance, it’s common for little kids to call other women mommy; even if they know she’s not their mom. This is a bit like that.
When I was eight I was chased by a big, scary dog. For years I avoided yards with dogs of all kinds. This is a normal defensive behavior. When your ex lets your daughter down she is sad. As a defensive reaction she pushes away the things she associates with this kind of sadness. In this case, it’s men.
The message she’s getting is that men can’t be counted on. Your partner can help change that perception. When she pushes him away he should not take it personally. But instead reassure her. He should give her lots of hugs and let her know he knows she’s having a bad day, etc. More importantly, he should make a point of never letting her down.
Throughout their lives, girls look for examples of the kind of guys they should have in their lives. If the only examples they find are unreliable men, they’ll accept unreliable men when they become adults. When they have at least one strong, reliable man in their lives they’re more likely to seek 
the same as an adult. She needs to know that not all men are unreliable.
Luckily this is only a temporary thing. She will eventually come around. He’ll help her avoid sadness by giving her an extra set of loving arms to run into when she’s sad.