I really feel this pain, I've always had a problem with the last third of a larger enema coming out. I developed a decent prolapse fairly easily, and I've always wondered if that had something to do with it. My partner can't prolapse, and his enemas are really straightforward. If he can't get it all out, all he needs to do is stick a toy up his ass and everything gets moving again. If that doesn't work for you, from my struggles I suggest:
- don't sit on the toilet, get up and move. Some actions will stimulate your colon letting go more than others, for me bending, picking things up, or walking up stairs is effective
- palpitate your lower colon/sigmoid. I'm relatively lean and can even feel the water that is trapped and almost "massage it out" sometimes
- give it time. If I'm going for a full clean out/depth play where I really can't have any mess, I spread my enemas out. I'll take about 2L fairly close together with the second one being when I feel like most of the water is out. Then I wait a few hours. I figure if I have trapped water it's going to be lower in the colon, sigmoid/descending area. Even if I can't get all the water out, I can slowly fill up (usually with one litre) and at least get the dirty part out, the stuff that is sitting low enough in the colon to be a part of the water that is released. Sometimes I'll do this second part a few times, and wait a few hours for the rest of the water to be absorbed. It's always worked for me, but it may not work that way for everyone.
This ends up being very labour intensive and pretty much requires a day off from university or work for me, so if you struggle with enemas, and want to play regularly, you sometimes have to ask yourself if for ease of play, the situation you're playing in (alone, with another(s), big session, or just ram your ass after a shitty day) and being kind to your digestive tract, if you're OK with having a more shallow clean/risking a bit of mess that you can rinse out if it happens