There are 2 major sources of incompletable mining missions:
You cannot use tech 2 mining or any instrument - including mining drone II - that has ‘residue’ in a mining mission.
If you run mining missions you must have exactly the amount at the site. The agent will not accept ‘and I had residue’. If losses happen, there is no possible way to complete the mission (although keeping the material after non-completion and you can use the material to make up the difference in the future )
If you mine with drones with a filled hold they jettison their cargo AND THE ORE IS GONE FOREVER
If drones return and their is insufficient space to store what they have mined they discard the remainder permanently which result in an auto fail on mining missions
This is not quite true - I use T1 drones extensively in mining missions. It takes discipline to ensure T1 drones always have a place to unload, but accomplish this and missions are reliable and completable.
It used to be that the site respawned the next day. That as unmade in an update more than 6 months ago. Any loss (including theft from another player) and there is no way to replace the missing ore. (Stockpiles from previous failed missions excepting and excepting blackmail/extortion payments to other players for extra ore). The only recourse then is to beg favors from a GM for a mission reset.
Mining missions aren’t quite protected, but the stories/history I have been able to track down on ‘balance testing’ for the creation of null and low security specific minerals was heavily influenced with these high security missions being effectively untouchable. Only two capsuleers have ever been attacked in a mining mission - in 2006 and this recently. Both were human trafficking targets. If the game-wide moratorium on attacks on mining mission sites ever changed, contracts and promises between CCP, CSM, and others may get called on to ensure CCP re-evaluates the entire low/high/null security mining system.