Just like the combination on a safe has to entered in the correct order, you have to play the moves in a combination in chess in the correct order.
Sometimes you see the moves which win, but realise that they allow the opponent to defend if you play them in the wrong order.
So if your intended combination doesn’t work, try to see if changing the move order helps.
In this week’s problem, White can checkmate Black. White has a choice of four tempting checks. They could all be winning moves, but there is only one winning line. Which order does White check Black in to force checkmate in 5 moves?
The solution to last Monday’s problem is that I played Ng4 which left White in serious trouble. The threat of Nxe3+ virtually forces White to play Rxd8+. When I recaptured with Rxd8, White was left with no good defence against the new threat of Nxb3 followed by Rd2+.