The approach you use when a relationship ended too quickly, someone who lost interest after just a few dates, or when things were just beginning to get serious is different from one you use to attract back someone you were in a “relationship” with.
If you approach things from the “get your ex back” perspective, you will get a lot of unnecessary resistance because the other person does not consider themselves “an ex”, and may feel pressured to act like an ex.
They may not understand why you are saying the things you are saying, or doing what you are doing to attract back an ex because in reality there was never a break-up to begin with. Any move you try to “get them back”, only makes them pull away even further.
And no contact is not going to change the mind of someone who had limited time to know you but prematurely made a decision based on limited information about you. If there is no opportunity for them to interact with you some more, they’ll think they were right to end things.