WW1G can be incredibly frustrating at times so you're not alone in thinking this. Because it is so detailed it requires us Players to pay attention to the details lest we find ourselves behind the situational awareness curve. Add to this there are some undocumented "features" that can blind-side you on occasion and a combat system chock full of subtle modifiers that can appear to deliver odd results at times.
However, these aspects of the game all combine to produce a very reasonable facsimile of the strategic issues that faced the supreme commanders and political leaders of the day. A much more complete game than the competition as far as I am concerned.
This stack ended up retreating to an area right behind (adjoining) it HQ. The next turn it had the little telephone symbol (out of command range I think) and I could not move that stack anywhere for the rest of the scenario. It refused to move to any area - even an adjoing area away from any enemy.
WW1G is based on a board game so often there are die (or dice) rolls required to make certain things happen. One of them is having the ability to move when out of range of the Army or General HQ. I have never found what the enabling roll for this is but there is one somewhere and luck, good and bad can drive whether the unit moves or not. If the Corps that is out of command is not HQ specific it may be detached and become a "independent" detachment, i.e. "A Detachment of France". This also works for Minor armies, sometimes the Romanian or Bulgarian forces can fall out of command control but they can be detached. Am less certain about doing this with the Corps of the major powers though, at least those labelled with their Army HQ designator. In the case that you describe I would not like to guess what happened without seeing a screen shot, sorry.
To create a national detachment is basically done in the same manner as creating Army detachments: right click and drag a short movement leg (it will be Red) and release; this should open a small window with various options. Select Create Independent Detachment and you're done. Independent detachments may always make legal moves but may not normally attack. They may stack with Army Detachments (up to stacking limits) and will defend normally if their province is attacked.
Stacking limits can be a pain as they are always in effect and may impede an otherwise legal move. Stacking limits varies with terrain so beware, this is another one of those little details that can cause grief if forgotten, particularly when the action move from plains to mountains and vice-versa.
is it OK to use your navy mod - at one point you were posting that it was causing crashes in the newer releases - the mod emulator that I used to install your mod - will it handle simultaneously installing your leader mod - you made a point of stressing only one mod at a time in one of your posts - can I do both your navy and leaders mod?
Should be, the error you mention was due to sloppy editing of the Units.csv file on my part in an early version that's been fixed. You may activate both the Leaders and Naval mods together but you may not activate two or more Naval Mods since the Units.csv files are overwritten and scenario specific. My mod list is Naval Mod, Nef's Sounds and JMass' Graphics. Plus I have tweaked some other files (including Leaders.csv) and events to taste.
I have tried this again (that and an interest in WWI - played CTGW and really liked it - read a book on WWI
I have somewhere over a hundred books on the WW1 era and have read many more but an excellent, very readable and insightful new work is A World Undone; The Story of the Great War 1914 to 1918. One of the best one-volume histories available in my opinion, less biased than many of the so-called classic works by Keegan and others.
Have rambled on a bit but good luck, help is out there and questions are always welcome.