The phrase "isolated instances" means that something happened only a few times or in a few specific situations, and not as part of a larger or ongoing pattern. Full definition
That is impossible in biology; we can not isolate instances of «evolution» in the laboratory and test them. (firstthings.com)
Test scores only improve in isolated instances, unless there is a school culture where asking for help, collaborating, and learning from each other in order to improve instruction becomes the norm. (naesp.org)
These are not isolated instances nor are they limited to «mom and pop» charter schools. (washingtonpost.com)