The important thing is to get your child to think about the words and for them to say the words to you very clearly using their lips and tongue to make the sounds of the letters that they will need in the word. One of the secrets of learning and remembering spellings is to say the words clearly moving the tongue and the lips.
Next see how many syllables there are in the word. Each syllable must have a spoken vowel sound. Don’t forget that ‘y’ is sometimes a spoken vowel in words. It acts as a vowel in the word baby when it makes the ‘ee’ sound at the end of the word and in sky when it makes the ‘eye’ sound at the end and in hymn when it makes the short ‘i’ sound in the middle.
Be patient, just because you have taught the word it doesn't mean it has been learnt or remembered.