First of all, a 2-quart enema is not really a large-volume enema. I know the size of people's colons vary significantly, but I'm also convinced that most people can take at least 3 quarts, albeit with significant cramping. Large volume or not, I've been doing enemas for a long time, and it sometimes does happen that after an initial expulsion it suddenly stops. No real urge in my gut to get rid of the rest, although I know there's plenty more water and gunk to be shat out. If I strain, it will come out in dribbles, maybe a squirt here and there, but no real satisfactory "gush" to empty me out. Kinda frustrating when it happens and the enema feels stuck.
Through the years I've attempted to read up as much as I could about this phenomenon (and it was really difficult to find much useful info), so I mostly had to take what I read from other people's experiences on a site such as this, try it out and then decide as to whether it really held true for me. I believe there are 2 "opposite" reasons for a stuck enema to happen:
A. The colon becomes overly relaxed by the enema (temperature or whatever is mixed up in there) and peristalsis is suppressed in some way or another;
B. The colon gets "knotted" (for the lack of a better word) by a kind of spasm in certain places (particularly at sharp bends in the colon like at the rectosigmoid junction), and the enema won't come out until this muscle contraction releases.
What to do about it? I believe there is a 2-fold approach which should work most of the time (it does for me, at least):
1. Use gravity to get the enema into the descending colon (your left side) by lying on your back, pushing your lower body up with your legs so your body rests on your shoulders and feet, with your buttocks raised as high as you can. Massage the water on the right side of your belly in this position (which is downhill) towards your midriff. Then you have to perform a bit of a contortion. Rotate you body towards the left, while keeping the right side high and dropping the left side down, until you lie completely on your left side. Pull the legs up, and remain in this position for a while. Most often you'll feel and hear the water gurgling towards your left side. Then get up while in this position (don't roll over on your back again) and go for the toilet.
2. Get yourself in the best expulsion position, which I have found to be either a squatting position (get a footrest of some sort to raise your feet about a foot off the ground while sitting on the toilet), or simply by bending forward as far as you can in the sitting position, grabbing your ankles or touching the ground. Don't be in a hurry here, it may take some time but this is a particularly good way to go from stuck to a gusher. You may have to repeat these steps a few times, to get it all out.
Let us know if this works out - good luck!