A level 5 trainer is a level 5 trainer, no magic beyond that. If you have a meaningful training program, you will see improvements in your players' skills. Changes in DMI (Deliberately Meaningless Index) are not a good indication of the effect your trainer has. A drop in DMI is probably caused by a drop in game shape.
If you are unsure how training works, spend a little time looking at all the training threads in the help forum.