So many different people and systems are involved in handling passenger luggage by the airlines that it is hard to keep track of. Each airline has its own way of doing things but the basics of each system are the same: check the bag to the correct destination on the same flight or series of flights as the passenger and have it on the claim belt no more than 20 minutes after arrival.
At connecting airports most bags run through the main baggage system unless they're running short of time when "hot" bags are transferred directly from one plane to the other. From A to B one bag can touch a dozen pairs of hands counting the ramp crews that load and unload them. Given so many moving parts one thing I have not seen is a listing of how many ways the system can fail. Here are a few of the most common.
"FTL" - Fail to Load. Pure negligence and nothing else, the airline simply did not load the bag. Whatever the explanation and however human and well intended they wanted to be it is their problem to fix.
"FTX" - Fail to Transfer. The handoff between airlines did not occur but the last airline you flew has to clean up the mess even if they never got the bag. They try hard, though, because what goes around...
"Short Check" - You're going to Fairbanks but the tag reads only as far as Seattle where the last transfer took place. This can happen if there is more than one reservation involved or if the originating airline does not have a record of onward travel. Show proof of your entire journey at check-in and read the claim check to make sure the bag is in fact going where you're going.
"MisLoad" - All of the connecting bags were loaded in Cargo Hold#4 but for some reason your bag was loaded in Hold#2 with the "locals" only going as far as the next stop. It may even end up on the claim belt if it is not noticed at the plane during off-loading. These are among the easiest to trace and fix.
"MisRoute" - Most airlines have more than one way to connect traffic. If flights are leaving to Dallas and Chicago fairly close to each other, either city can get the bag to Los Angeles. If it's not on one carousel it might be on the other so check with the agent before storming off. It could be only a few minutes behind or it might already be there. These get trickier if Dallas is the only direct way to get there.
"Late Check" - Entirely your fault, plain and simple. You show up late and some airlines make you sign a waiver of responsibility if the bag doesn't arrive when you do. They'll try their hardest but here again some won't even let you check in at all these days for security reasons.
"MisCon" - Real simple here, you missed the flight. Any number of reasons, air traffic control, mechanical, gate occupied, yada yada. Miracles do happen but each circumstance dictates how long it may take, especially if it's compound like bad weather on top of cancellations or diversions followed by mechanicals but by and large the bag gets on "the next thing smoking" right along with you.
"WB" - Weight and balance. Sometimes there are simply too many bags or too much weight for one airplane to carry, especially if that plane is full. Twenty marines can show up with 100 bags weighing 60 pounds each, on average. It is simply too much for a puddle-jumper between Atlanta and Dothan even in good weather.
"TSA" - If your bag raises suspicion for any reason whatsoever it might not make the first flight or any other on your journey. The airline suffers the bad mark but think about every single item you pack. Large spray cans, bowling gear, firearms, tool kits and solid plastics might not get confiscated but they will hold things up.
"TagOff" - Not much to explain here, the tag fell off. Make sure the agent secures the tag firmly but it can still get torn off by accident during handling. Here comes the call for name tags as well as some kind of identification inside the bag itself.
Garden variety things happen every day. Sometimes it helps to air some dirty laundry in public so everyone has a chance to travel clean.