I'd work hard on shifting the release to be the reward for her, especially if she's dragging without food rewards being imminant. And is her thyroid OK? At least this is what I'd try. Your pardon in advance if these are things you are already doing <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
Treat randomly throughout training sessions so she can't always predict whether this is a treat-winning exercise or not. You can always gear her with some treats for fast sits, speak, catch, or some other quick, fun, thing before you, go into a motion exercise (where you might not reward again until the end).
As an alternative reward, I would release her from exercises more promptly and more often for good performance. Like when varying rewards while heeling, release her very promptly for quick sits, or quick starts from halts, a very few quick steps when heeling, by releasing her imediately and totally.
Do it very often in the beginning of getting her "bouncy" when varying it with her other treat rewards.
Also practice making her wait for her rewards. Like ask for eye contact, get it, release and praise and then the food.
Make the release special, she gets to bounce, sniff, etc. and mess with her, scratch her butt, whatever she likes. You won't have food in the ring, but you will always have the release! The release in itself is really motivating for a lot of dogs.
Keep her sessions really short and "up". Rotts are really smart, but they get uncomfortable easily -- too hot, uncomfortable surfaces even when "relaxing".
When she's moving slow, have you "bounced" her head like a basketball? I don't mean hard, but a pretty good push down and take your hand right off, and then laugh with her? That sometimes will brighten them up if they are lagging. If it does, I'd release as soon as you get it in the beginning, so she knows you're looking for her to perk up and will reward it.
Also 'cause rotts are so smart, make real sure you are not asking her to repeat the same things over and over in a session. Some rotties really stress when you repeat things (like the same *whole* heeling pattern over and over because of, say, a slow sit). I think they turn it over in their mind trying to figure out what's wrong, can't isolate it for themselves, and decide the whole game stinks. Rotties are really into integrity and "fairness" in work.
You need to help her. If she's blowing parts of an exercise, break it down, work that small part asking for a little more each time, and then put it back in context. With dogs like rotties, it really helps to plan what you want to accomplish in a session so you can keep it short, focused, and put her away while she's feeling good about herself.