Some children do struggle to learn and the best way to deal with this is to do little and very often. Write the focus phoneme on a strip of card, all the way round. The child could even make their own bracelet. Tape it to the child’s wrist and during the day keep asking what the sound is. Eventually they will remember it. You might need to do this over several days. They can wear their bracelet at home too and ask mum or dad to keep asking them what the sound is.
Write more than one sound on the bracelet or write simple words for blending.