I haven't dealt with this in a diesel yet, but on my brother's mk5 golf (gas). I had borrowed it on a roadtrip to NC to visit a friend and on the way back all of a sudden I couldn't get it into any gear. I called AAA and the tow truck driver and I bled the clutch together. That was a bandaid fix that held me together till i got home and for about another month. Then finally it really went. Turns out the clutch was worn through to almost threads AND the SLAVE cylinder was in shambles. (I can post pictures if you want me to)
Gary...
I just had the system bled new fluid put in so Cant believe any air is in there at this point. The age of the clutch not sure... I picked up the car for my daughter at 198K now has 220K on it. Bought it off a little dealer so not much background info.
If what you did so far doesn't do the trick, I would take the slave cylinder out and inspect it. It should be external, and if it's like the jetta tdi or golf's layout, you might need to take out the battery housing or air filter housing for better access.
Then think about a new clutch. I try to work cheapest/easiest to most expensive/time consuming when diagnosing problems.
In my case the new clutch/flywheel and slave cylinder did the trick.
All the gears have there moments shifts perfect when the car is not running... seems to shift better at lower rpms 2k verses my normal shift point at 3k. Truthfully my jetta is pretty darn notchy not much different than this car is now but my daughter wants it back to the way it was... I hardly noticed it until she really pointed it out to me because I am so use to driving my jetta anyway.
I can understand that too. We get used to how things are/were. Again in my case, the old clutch was grabbing very late on the clutch pedal. (as in higher up on the swing). I was accustomed to this because every manual VW i've driven was like this (all used and roughly 10 years old). After the clutch slave was bled however, it was engaging really close to the floor. So much so that I stalled several times at first because my left foot was rushing to just about where it used to engage (muscle memory). Once the clutch was replaced it remained down low... Took a about 100 miles to get used to.