Sometimes it has more to do with relaxation than volume. When I am tense, my body refuses to cooperate and no amount of enemas will result in clear returns. It's also difficult to retain water for any length of time. If you're stressed out today, try deep breaths and meditation or anything else you do to relax while you're taking your enemas.
You may want to try different positions until you find one that works best with your body. If I'm having a series, like to start out by lying on my left side. Then when I start to feel full, I'll lie on my back and let the water spread across my belly. I sometimes lie on my right side a while then get on my hands and knees for a few. It helps to move the water through the digestive tract so it can hit every nook and cranny.
Having a partner administer is ideal as they can do all the work and you can focus on lying there and relaxing. But I realize that it is very difficult to find someone. Just if you're lucky enough to have an enemate on hand, it helps.
Sometimes it has to do with the consistency of the stool. If it is too hard and you are dehydrated, it sometimes adheres to the intestinal walls. Massaging the belly as you take the enema, using a colon tube to get a high enema or using an oil enema to soften the stool may help in that situation. But be warned, oil enemas can be messy and you'll need to take at least one soapy enema to wash out the oil. Otherwise, you could leave oily stool trails wherever you go.
If your stool is too soft, it takes on the consistency of toothpaste and you just get an endless stream of it. Sometimes if it is too soft, it seems to act like teflon and water just rushes by it and has no effect whatsoever. Adding more fruits, vegetables and fiber to your diet and drinking plenty of water can improve the consistency of your stool and make your body run like clockwork.
Sometimes an irritant, such as glycerin or soap can cause an enema to be more effective as they cause the walls of the colon to produce similar movement as when you need to defecate. I'm not fond of irritants so I very rarely use them. A saline Fleet enema is the harshest enema I'll take. But I'm a wuss. If you don't mind pain, burning, cramping and explosive outputs, there are plenty recipes to try.
I hope one or more of the above ideas helps. Good luck.
Dianne