Jahn-Teller effect is not observed in high spin complexes of:
d7
d8
d4
d9
Jahn-Teller distortion is usually significant for asymmetrically occupied eg orbitals since they are directed towards the ligands and the energy gain is considerably more.
In the case of unevenly occupied t2g, orbitals, the Jaha-Teller distortion is very weak since the t2g set does not point directly at the ligands and therefore, the energy gain is much less. High spin complexes: