You will see this in almost every classroom. A child gives you the sound of s, then a, then t, and the moment you ask what the whole word says, they go silent or blurt out a completely different word. This isn't laziness and it isn't a lack of effort. It's a developmental stage called blending, and it's far harder than remembering single sounds, because the brain has to hold the first sound in mind while it works its way through the ones that follow.
Technique 1: Stretch the Sounds, Never Stop Between Letters
The main reason blending fails is that children say the sounds with gaps in between: s, stop, a, stop, t. By the time they reach the last sound they have lost the first one. Get them stretching the sounds together instead, slowly, almost singing it: ssssaaaat. Then speed it up bit by bit until it turns into a word. There's nothing at all to remember with this method, because the sound never breaks.
Start with words whose first sound can be stretched (s, m, f, n, l), so sat, man and fan. Leave words that begin with a stop sound, like cat and top, until later, because those are hard to hold.
Technique 2: Go Back to Blending by Ear, With No Letters
If stretching still doesn't get you there, take the letters away altogether. Say the sounds out loud one at a time, m, a, n, and ask what word you just said. Let the child close their eyes and listen if that helps. Once they can answer easily by ear alone, bring the letters back in. The usual problem is that a child is carrying two jobs at once, recognising letter shapes and blending sounds. Take one job away and it clicks.
What Not to Do
Never make a child who can't blend yet read aloud in front of the class. Nothing makes a child give up faster than embarrassment. And don't add more words. Cut back to three or four that reuse the same set of sounds, over and over, until it comes. One child can take months longer than another, and that's completely normal.