Nothing to do with luck, Steven had an alibi and Luke didn't.
Not quite! Both had alibis, each of them (obviously) dependent on the word of other people. Luke had an alibi from the first day of the investigation - his mother. His brother told police on day 5 of the investigation that Luke made the tea that evening - in his first statement on day 3, he couldn't remember much about the Monday evening.
Steven had no alibi until day 12 of the investigation. In fact, until then, he said simply that he'd gone home from Janine's "in the afternoon" and had his tea, then gone back to Janine's later. The story about them going to Kelly's father's together for tea at about 4.30pm and returning at around 7pm didn't emerge until day 12.
Kelly's father was able to remember, 12 days later, exactly what they'd eaten on the night of Monday 30th June. No-one thought that was suspicious.
Shane didn't remember what he'd had for tea 3 days earlier - it was only when his mum reminded him on day 4 that he went back to the police to correct his statement, but that
was considered suspicious.
So, really, it came down to what the police chose to believe about alibis, rather than who had or didn't have alibis.
Here's a little challenge - what did you eat on Monday 22nd October, with whom, where and when and can you prove it?
What did you eat on Tuesday this week, with whom, where and when and can you prove it?
I'll go first - no idea and no idea. I cheated and checked my diary, but I'm not in the habit of recording what I eat. I can tell you I was home alone both days, so even if I could remember what I ate, I couldn't prove it.
Anybody else?