I ran into that with Dragon Valley. I had purchased it online (I think from Amazon). I had it registered, but when I brought it up, it said I needed to buy it. It turns out that I HAVE TO go to the Sims 3 store - hover over the Store tab, then My Sims account, and then click Purchase History. I have to use the actual download button there to download and install it. It's frustrating to have to do this, but it's the only way I have access to download it. Hopefully, that is the reason you can't download them from their store pages. I recommend storing the Sims3 files on a thumb drive or somewhere safe for those items. Copy the files from the Downloads folder once they are downloaded and installed.
@kiwi84 - Here is an alternative to going through Purchase History. Valgon57's thread Store Mega List for Extra Quick Redownloading gives instructions which allows you to download and install without going into Purchase History.
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