i do get half evacuation and its very annoying, disappointing and a sense of failure. and like yours, i have to wait till the next morning for another 'go'. i have to wait till its urgent and then i can go, because the bottom can open up more than if you had little pooh's in there. the spincter does need good stimulation of the proper size mass so as not to pinch/or cut in half and not be done properly.
Sorry for being so graphic but its what i have learnt.
I have got my son's stepper so he can use the toilet and reach taps, and i tell you that i use it to put my feet on when on the dunny and its realy good for opening to bowels, it straightens out the bowel for a better result. we are really not meant to sit 90 degrees, like we do on the toilet, to open bowels. we really need to squat, because it straightens out the back passage.
IBS has pain and cramps and constipation alternating to diarrohea, so im not so sure. you may have parcopresis, i think i do too. im too afraid of living with my bf or having him stay over. i dont know how other people can just live normally and do their business with lovers around them. I wish i was normal.