I've experience something similar with a Sim. She was in my library, but her photo didn't show up. What I had to do was place her on a lot and re-save her to my library. I'm not sure if this is the issue or not, but I thought I'd mention it just in case.
Try downloading the Sims with a fresh Sims 3 folder. To do that, move your Sims 3 folder to the desktop. You won't loose anything. It will be safe on your desktop.
Open the launcher, this should create a new Sims 3 folder. Log into the Sims 3 site and try to download and install the Sims you want.
Click the downloads tab in the launcher, Click Select All and Install.
There have been reports of players having issues with the exchange, if the above doesn't help. You may want to try posting about your issue at Answers HQ -
https://answers.ea.com/t5/The-Sims-3/bd-p/the-sims-3