OK you found a lurking SALT.
If you don't mind my asking, which sound are they working on with her? Some sounds are aquired later than others, which may explain why she has not been making progress. Is it just sounds she has difficulty with? How are her sentences and understanding of language?
Obviously I can't comment specifically on your dd's concentration skills, but it seems obvious that if all the other children in the group are older they will have better developed attention skills.
It is standard procedure in SALT to liaise with all professionals involved with a child and to give advice where necessary. Your SALT should be talking to the nursery at least, and sending them progress reports. I realise many SALTs are incredibly busy but IMO it is not good practice to work in isolation with a child without considering all the other environments the child is in during the week. When I give breaks from therapy I always give games to the parents and school/nursery to do in the meantime. Many schools say they do not have time to do extra work with children so your dd's nursery should be given material if they are willing!
Lastly it is not helpful to "correct", if that is what you were told to do. I advise parents to repeat back the word accurately, but in a positive way so the child doesn't constatly feel like they are making a mistake. If a lot of work on a specific sound has been done, you could remind gently about what your dd needs to do with her mouth.
e.g. if a child has problems with "k", and has been practising using the back of the mouth, you could say something like "oh I think I hear a back sound in "car", do you hear it?" etc etc
Sorry this is long, hope it helps.