Brief Answer:
it could be due to sticky or dry vagina, use lubricant

Detailed Answer:
Hello dear

i had gone through your question diligently and i can understand your concern
there are number of reasons for falling off condom during sex.
1. erectile dysfunction (incomplete erection of penis) which can be related to excessive anxiety or stress
2. Quality of condom as cheap quality of condom can slip off easily during strokes
3. Less vaginally secretion (she might not be getting proper wet) so you can apply any lubricant to overcome this
4. Sticky or narrow vaginal opening

I would advise you following guidelines
spend some time in foreplay and get good erection
use condom properly, pull it over the entire length of penis
use a lubricant on the condom as well vagina
you can change the position of the sex preferably doggy style or from behind the girl where you will feel less resistance

Hope you will find my reply informative and helpful pleas don't hesitate to ask for more clarification I will gladly answer you
best wishes

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