Discrimination Between the Sounds in Words:
First, try to get them to hear the difference between two meaningful words that are minimal pairs with the only difference being the sound you’re trying to get them to discriminate. For example, if you’re working on /t/ and /k/, try “tea” and “key”. Show them two pictures of the words and tell them what they are (“this is key, this is tea”). Then, say “point to key” or “point to tea” and see if they can point to the correct picture. Hide a piece of candy under one to make it more motivating. They can only eat the candy if they pick the right picture.
Discrimination Between Sounds in Isolation:
If you’ve been trying words for a while and they still can’t do it, try doing the discrimination in isolation using just the written letters as the choices to choose from. Have the child watch your mouth to use visual cues to differentiate between the two sounds. Use candy again as a motivator if necessary. Once they can do it by watching your mouth, use a speech screen to cover your mouth and get them to the point where they can hear it without using the visual cues.
Have you checked the hearing?
If they still can’t tell the difference after all of that, perhaps the hearing should be checked again…just saying….
If hearing is OK but they really are still not getting it, I would try doing auditory bombardment of the error sound with amplification and visual modeling (watching your mouth). Have them attend to your mouth while you say the sound in isolation as well as in single words while overarticulating the target sound. Do this for a while and then go back and try the discrimination activities again. Perhaps even try the discrimination activities with sound amplification.
Use a simple app like this for some quick sound amplification: https://itunes.apple.com/us/app/hearyounow-your-personal-sound/id569522474?mt=8