Most procedures I am aware of call for 'pink color stable for at least 30 sec'.
Permanganate slowly decomposes on its own - that can be the problem.
It is also possible other substances present in pills are reducing permnagante - there are digestion procedures you could use (like remove organic substances by boiling the solution with some strong oxidant, remove the oxidizing substance by drying the solution in the presence of concentrated sulfuric acid - till you have only solid iron sulfate left, then reduce iron to Fe2+ - I can be wrong about details, but that's the general idea).
I will forward the question to CHEMED-L.
This is not a titration experiment, but you may try to check this paper from Journal of Chemical Education:
Colorimetric determination of iron in vitamin supplement tablets. A general chemistry experiment
Atkins, Robert C.
August 1975 p 550