Given your s-l-o-w system, they may be breaking down and absorbing, which could be why you don't see them. On the other hand, maybe they are indeed getting hung up in a twist or turn in the intestine and causing things to move more slowly.
I cannot take them - they just don't digest at all for me. Same with Calcium pills that are not liquid gels. Amey, can you try a liquid form of potassium and see if that changes your output?
REmember, we are all a little different --- trial and error!!