It's the same process whether she's in heat or not. Heat tends to make her horny and looser inside. If you're unable to do it then perhaps you're too big for her or she just will not want to have sex out of heat. These are things you can't change. The best you can do is have sex during her heat, and then after she goes out of heat wait 2 weeks to have sex again for to help prevent infection, then see if you can have sex with her again right away. She might still be loose enough from the last sex she had to continue out of heat.
What prevents you from having sex with her out of heat?