There are some times when a student is working on fixing a mistake and things just don't seem to be going anywhere. Here are a few reasons that this may happen.
1. The student stops imagining the corrected sound or feel once they start playing, and just reverts back to the mistake
2. The student does not have the precise imagination of sound or physical feeling in their mind that will make it possible to correct the mistake while they are playing
3. The student has not found the proper thing to imagine (the sound of a steady beat, a different feeling in the rhythm of their hands, etc.) that will correct the mistake
Most students can get back on track with a reminder to do #1. Sometimes #2 is simply a matter of the student paying more attention to the correction task. And #3 is most typical if the student is trying something new and/or has not thought through exactly what it is about the music they are trying correct. Once a student becomes more aware of how their mind is working while fixing a mistake, they can start using these solutions in their practice time.