Thank you for asking. I can understand your concern. Let me first inform you that the Erection depends upon many factors. Most importantly, adequate arousal and excitement is required for complete erection. It is possible that when you are with your girlfriend, the arousal levies and comfort levels are good and these are promoting good erections. But when you are with different women, there may not be a similar level if comfort which is what is preventing good erection. For good arousal and erections one must even emotionally attached to the partner only then one can enjoy good sex.
Next Steps
I think what is happening with you is very normal. I don't see any reason for being worried. For more information regarding this you can consult directly here on practo.