The fact that you get frustated when you don't get sex means you're probably not having sex enough, hence you get too eager once they are inside you apartment. Girls smell you eagerness and leave...
Remember that in general a woman wants to have sex with if she comes to your apartment. So it means you fucked it up!
Next time when they arrive at your apartment, even if you've already spent 2 drinks at a bar with her, go both sit in your sofa, take your time, drink something at ease, escalate with kissing but always pull back first. Make her want to have sex with you more than the other way around. Then go in for the kill.
Remember that in general a woman wants to have sex with if she comes to your apartment. So it means you fucked it up!
Next time when they arrive at your apartment, even if you've already spent 2 drinks at a bar with her, go both sit in your sofa, take your time, drink something at ease, escalate with kissing but always pull back first. Make her want to have sex with you more than the other way around. Then go in for the kill.