That's why it is so important to have the soap well mixed by the time you pour it in the bag. I have a seal-able jug that I fill with water and put a bar of Ivory soap in. I shake it up when I fill it, and then I stash it away in the back of the closet. When I am planning an enema, I shake the bottle up at least the night before, and again right after I carry it into the bathroom to mix the enema.
Also, that is a good reason to use the closed-top, Hot-Water-Bottle style bag, instead of the open-topped bag. That way, you have to pour the soapsuds in, then fill it the rest of the way with hot water. You want the entire mixture warm, not hot. Most people, myself included, automatically, without giving it much thought, will pick the bottle up by the hook, and the end of the hose, and carry it to the enema bed that way. That helps, because it puts the soapsuds on top, where they will dissolve into the water.
I always lay the enema on the bed for 5 minutes or so, while I am checking and making sure I have everything right. Then when I do hang it, it has the soap pretty well mixed with the water. Then I get into position, and wait another couple of minutes, to make sure I'm relaxed before I do the insertion, then I take another minute or two, and again make sure I'm relaxed before I open the valve.