had a scania today and all was fine with the gearbox and then suddenly couldnt get any gears at all then suddenly it worked fine ,this must of happened 20 times or so , woulnt of thought clutch as it would of been all the time ,some kind of selector problem , just seemed really strange how it would work for 10 mile or so fine then nothing ,unable to get in any gear then all of sudden droped in and worked fine for a while 
any ideas ta
could be down to an airleak
were you rushing the range change? changing gears too fast will leave you in a neutral range, until you go back to neutral and then back into gear
If not,
It could be a loose bolt on the selector, there is a half moon shaped slot in the linkage on the nearside by the gearbox, sometime it comes loose and slides to the wrong postion,
Or
The bolt at the front of the gear linkage has started to fall out, they have a habit of undoing themselves
Discopetes first reason is the most likely cause in my experience, positive gear changes and range changes are the key. When you come to a stop just double check you have a gear B4 moving off again.
All the Scanias I’ve driven seem to have a gear missing 
Seriously though, I have been caught out on them by rushing the range change.
as i drove another one today and had no prpblems i dont thing it was me ,also driver who had it last night rang in saying theres something wrong with gearbox one minute i can get gears next i cant
sounds like the neutral valve to be faulty