Jim and Kbay may already know this, but something that not everyone is aware of...
In speaking with FSS guys over time, especially when plans don't get sent over to the "real" system, I believe the main problem is the lack of consistency with how a real flight plan gets filed and how we do it in the military. The system can't handle multiple legs like we write on a -175. Instead, the way it actually deals with it is to have two (or multiple) flight plans. So when you put a terminal delay and then continue to your destination on a -175 (ie, the format you always have to look up in the GP), that's actually two flight plans, even though you write it as one. I'm pretty sure that the junior ACs don't really understand that and that's a big part of why things get lost. Then again, a lot of times I've seen where one simple VFR leg won't even make it in there, so...who knows.
One other thing to know is that all the crap you put on your -175 doesn't actually make it over to the FAA except the actual waypoints. Putting delays and circle-Rs and VOID times (yeah, I know we don't do that anymore) aren't ever seen on the FAA strip system that the controllers have. Basically it's a waste of time if you're going somewhere other than military.
I'm sure HAL has a lot more info on this, but this is what I've witnessed.