It's probably a bit of both.
She may be one of those types who isn't able or willing to adapt to changes well, or who can't manage her own time well, or who just outright doesn't want to put in a full day's work on a regular basis.
She also might very well be experiencing being pulled in literally a dozen different directions, where her workload is too overloaded, and it doesn't take a lot of people one supports for this to happen. All it takes is one or two "higher ups" who take advantage of haing an admin. where they assign tasks which are total timesucks for an admin's schedule, ability to come through on time with deadlines/deliverables, and things get thrown off-kilter...and then it snowballs, until no one's happy.
Also, when doing expenses, sometimes an admin. has to chase down the people she's supporting, for receipts. It's a PITA, especially when dealing with disorganized or disinterested "higher ups" who don't bother to supply the admin. with those, which are required, in order to enter-in and submit expense reports. The receipts are requested, the request is ignored, the admin. follows up, gets ignored again, then promised that they'll be sent, but they never are. The admin. has to hound the "higher ups" again for receipts, then finally gives up, and then has to play detective with each vendor (hotel, restaurant, etc.) to try to procure the portfolio/receipts from them directly. It's a total nightmare having to pull teeth, but this is what some admins. have to deal with, because the people they're supporting are cluelessly disrespectful toward those they deem to be "beneath" them.
So doing expenses for 12 people can be a lot, especially if even just a few of those people are as described above, and definitely if there's a lot of travel involved. But the admin. should advocate for her/himself as well, by speaking directly with the people they're supporting, and letting them know what they can accommodate in terms of bandwidth, time, etc. Ask them when they need something done by, instead of just saying yes, with no deadline. Communicate with them as you're working on something you need more information on to complete the task, and let them know you need to hear back from them by a certain time/day. Keep it all in writing, and pull that up if needed, to light fires under a55es!
This admin. might be overburdened, but she might be burning out, because the people she's supporting aren't helping her, in order for her to help them. Admins. are often underappreciated, but a lot of them (the good ones) are the glue that holds it all together!