Isuzu auto problem?

Hi , I have an Isuzu with the AW 30-40 LE transmission
forward gears are fine and all as it should be , reverse
is sometimes normal but often feels as if the brakes are being held on ( they aren't obviously ) when like this if you stop facing uphill the car will not roll back in neutral or reverse and will only move backwards under power in reverse or will freewheel if you switch off the engine , is there a transmission expert that can advise
please , regards , Tim. p.s, I'm a plant fitter so fairly
able mechanically but not with auto's!