You might have some bad cc, maybe attached to one of the lots you downloaded. Or there's something else in your Sims 3 user data folder that's preventing the new content from installing properly. Or the new content you're installing could be corrupt, either bad content or a bad download.
The easiest test is to pull your entire Sims 3 folder out of Documents\Electronic Arts and onto your desktop, open the launcher to create a new folder, then install the new content you've been trying to add. If it works—it installs, it shows up in-game, there's no extra content you didn't expect listed in the launcher's Installed content tab, and opening that tab doesn't crash the launcher—then the issue is something in your old user data folder, and you'll need to test to find out what.
How much custom content do you have installed through the launcher? With the lots you've been downloading, do you want to keep the cc, if any, that came with them? Would it be a pain to reinstall your existing cc? There are a few different approaches you can try, depending on whether you're willing to start from scratch and/or delete some content.
