There are a couple of exceptions to the free 4K upgrades. For some movies, studios have opted to make an entirely new listing for the 4K version rather than upgrading the existing version to 4K. Heat is definitely one example (the original HD theatrical cut is no longer for sale and was not upgraded to 4K - they replaced it with the director's cut that is 4K - If you own the theatrical cut and want the 4K director's cut, you have to buy it separately). Not sure about The Fifth Element.i bought The Fifth Element and Heat on iTunes (HD). Neither were upgraded to 4K, yet you can buy the 4K versions on iTunes.
If you are also linked to a Movies Anywhere account, that might also be causing problems. Movies imported into iTunes via Movies Anywhere that were originally purchased in HD at another provider do NOT get free 4K upgrades on iTunes. The movie has to have been originally purchased on iTunes.