Selecting 4 wheel Drive

Can anyone assist?

I have a problem selecting 4-wheel drive, in my R reg Isuzu trooper 3.1 TD.

Then you select the 4 wheel drive (Shift on the fly), when moving or not, It DOSEN’T activate.

If you switch the engine off and select 4-wheel drive it may activate, it may NOT

If you leave it in 4-wheel drive for a period of time like an hour and start the car it will engage.

Any suggestions please

Thanks JIM