This parable seems applicable here;
Mark 4:3-8
3 Hearken; Behold, there went out a sower to sow: 4 And it came to pass, as he sowed, some fell by the way side, and the fowls of the air came and devoured it up. 5 And some fell on stony ground, where it had not much earth; and immediately it sprang up, because it had no depth of earth: 6 But when the sun was up, it was scorched; and because it had no root, it withered away. 7 And some fell among thorns, and the thorns grew up, and choked it, and it yielded no fruit. 8 And other fell on good ground, and did yield fruit that sprang up and increased; and brought forth, some thirty, and some sixty, and some an hundred.
And Jesus explanation;
Mark 4:14-20
14 The sower soweth the word. 15 And these are they by the way side, where the word is sown; but when they have heard, Satan cometh immediately, and taketh away the word that was sown in their hearts. 16 And these are they likewise which are sown on stony ground; who, when they have heard the word, immediately receive it with gladness; 17 And have no root in themselves, and so endure but for a time: afterward, when affliction or persecution ariseth for the word's sake, immediately they are offended. 18 And these are they which are sown among thorns; such as hear the word, 19 And the cares of this world, and the deceitfulness of riches, and the lusts of other things entering in, choke the word, and it becometh unfruitful. 20 And these are they which are sown on good ground; such as hear the word, and receive it, and bring forth fruit, some thirtyfold, some sixty, and some an hundred.
So, what's the point? The point is, the sower sows the Word. He sowed it on good ground, bad ground, hard ground, thorny ground... regardless, the sower sows the Word. To apply it to this topic, at "mass evangelism" events, there are all types of "ground" present. But still, the sower sows the Word.
Jesus then explains why some people, that have heard the same Word, don't respond the same way, and sure enough, neither the method of sowing, nor "follow-up" was not on the list.
What was on the list was Satan stealing the Word sown from them at the wayside, affliction and persecution for the Word's sake, cares of this world, deceitfulness of riches, lust of other things.... THOSE are things that make the Word unfruitful.
It is not the sower sowing the Word that is the problem. In fact, Jesus never said that the sower should only focus on good ground, or that he should sow a certain way. But the sower sows the Word to EVERYONE.
Follow-up can help people to avoid the pitfalls that Jesus listed, but ultimately, it's up to them to decide if they want to be good ground, or if they want to allow other things to steal that seed away from them. It's not the evangelist's responsibility, neither is it the responsibility of the "follow-up" people. The sower has one job;
To sow the Word.