Not sure of a precise answer for this, but attitude comes into it a lot. I think every club has players like this and I see average standard batsmen do well in the nets - good technique, good shot selection, who then play totally differently in the middle - introvert and terrified of giving their wicket away.
Conversely, I see some batsman have a proper slog in the nets who then wonder why they can't buy a run getting bowled swinging across the line on low early season tracks!
There needs to be an environment where they feel they can play their shots, to a point, in the middle without feeling as though one or two low scores will see them criticised. A lot of encouragement in the nets when relevant, but also the practice in the nets has to be purposeful. I get criticised at club nets for leaving balls and defending more than most, but as an opener, that's my job. There's no sense in me trying to Chris Gayle every net ball for six when I won't play that way in the middle.
Encourage your batsmen, give them the confidence to play their shots but also make it clear that their net session should be reflective of how they play/how you see their role -and bat accordingly. Practice doesn't make perfect. Practice makes permanent. Perfect purposeful practice makes perfect. try to create a game situation in the nets for them - visualise fielders but encourage their productive shots. Have the skipper or senior player watch them for a bit and feedback, after every couple of balls or so. But keep it positive, with constructive feedback. A the end of their net, ask them how they feel it went and how they can apply the positives to their outdoor game.