Some children have difficulty developing individual sounds within speech. Generally these will solve themselves through maturity other children need support from a speech therapist, who will ascertain the precise nature of their difficulties and recommend the best course of action. In my experience the resulting recommendations either to hearing the sounds and discriminating and or producing the sounds in isolation moving on to blending with vowel sounds and eventually whole words. Black Sheep Press produce a range of great resources to support, but due to the generic nature often do not tap into the individual child's needs and interests, which we in the early years are expert at.
These resources were developed with that in mind, if a child is interested in threading make pictures with things beggining with that sound or make that sound to thread (in this example I use newspaper letters simply because the child knows their letters and interested in identifying letters in the newspaper. I am planning on doing this for all the letters as we now want to spell with them!). Play various games e.g. turn up cards and name upon threading, place cards face up and take it in turns to ask other to find "c" etc.Moving on from that posting can be a firm favourite, create a posting box to post individual items, letters in naming as you do so, initially starting with just the initial sound.
Remember just enjoy playing with sounds do not exert pressure, if a child says a word wrong repeat it back in the correct manner emphasising the sound that causes difficulty, e.g. "I want the tat" "Ok, where's the Cat"
There are loads of really useful ideas and resources on this local NHS Speech and Language site
The Inclusion Development Programme for Speech and Language is also an invaluable resource, especially for auditing your provision.
"Speech both conceals and reveals the thoughts of men."