Slow digestion. What could be the cause?

How is your fibre intake? Besides the prescription from your doctor I mean. There is a kind of a huge deficit when it comes to the population's dietary fibre consumption on the whole. You could try either altering your diet gradually to increase fibre to the recommended allowance, or you could change your supplement to a different kind. Linseeds (or flaxseed as they're sometimes known) are a fantastic addition to your diet, they're excellent for constipation

That being said, fibre can sometimes make people feel fuller for longer so the current supplement you're taking may be contributing to that fullness feeling. When I first started having trouble with constipation I raised my fibre intake and it made me worse, so its not for everybody, but its certainly worth trying because its unlikely anything terrible will happen as a result, more just wind/discomfort.

Failing that you could ask your doctor what he or she thinks about you trying something like lactulose, its a dietary supplement for people with chronic constipation (and sometimes diseases of the liver) and its usually safe to take long term (excluding any pre-existing medical conditions obviously). It might help your bowel movements to be more regular which might in turn help your appetite.

Lastly other things to consider might be whether or not you have a sedentary lifestyle (very bad for bowel motility) or if you might perhaps have any kind of food intolerance. Speak to your doctor about your options before taking any action, best of luck either way, for all of us its a process to find what suits us best :)