What if you try to bring a Jew to do a mitzvah and you do not succeed? What if the Jew you are trying to help walks away angry or laughs in your face and rejects you completely?
We can learn the answer to this question from a Jew who tries to convince another Jew to worship an idol. Even if he does not succeed, if he is convicted, he will be executed by the Jewish court.
Now, surely if Hashem orders a Jew killed for trying to convince another Jew to do a sin even if he does not succeed, how much more so is He going to give a reward to the Jew who tries to bring another Jew to do a mitzvah even if he does not succeed! Certainly Hashem Who wants us to help each other to do mitzvahs is going to reward the Jew who tries to help another Jew even if he fails.
But before you feel terrible for having failed, let's be sure that you failed.
Some time ago a middle-aged American man saw me asking someone to put on tefillin at the Kotel. The man I asked walked by without stopping. The American walked over and said;
"Don't feel bad when you ask a Jew to do a mitzvah and he refuses. Let me tell you a story. Many years ago I was walking down the street in Los Angeles when a young, religious boy asked me to stop and put on tefillin. It really bothered me. I yelled at the boy, 'You just lost your share in the World to Come. You embarrassed a Jew in public!' and I ran off steaming with anger."
"I walked fifty steps before I calmed down. Then I stopped and said to myself, 'How low have I come that I got angry when a Jew asked me to do a mitzvah.'
"I turned at that minute and went home and put on my tefillin and I've been putting them on every day since."
I bet, to this day that boy who tried to help him still feels bad for what he did. But not only will Hashem give him a huge reward for what he tried to do, he actually saved a Jew's spiritual life and he doesn't even know it.
The point of the story is; if you go out with a good heart and try to help a Jew to do a mitzvah, one way or the other, you can't fail.