ThruLines/ Ancestry not detecting obvious shared common ancestors?

I just got my AncestryDNA results back and have a ton of matches. I've been going through my matches, looking at how much DNA they share with me and with other matches, and using that information to piece together my family tree.

The issue I'm having is that Ancestry doesn't seem to detect some really obvious shared ancestors.

For example, I'll look at another person's tree and see that we literally have the same ancestors in both of our trees. I've even saved people and information directly from their tree into mine, but Ancestry still doesn't recognize that we have a shared ancestor.

Because of this, my ThruLines is pretty limited. Right now it only shows around 18 people, even though I have thousands of DNA matches on both my paternal and maternal sides.

I feel like with all of those matches and all of the public family trees connected to them, Ancestry should be able to connect a lot more of this information together. Instead, I'm having to go through matches one by one, figure out how they're related, and manually add everything to my tree.

Is there something I'm missing? Is there a way to get Ancestry or ThruLines to recognize more shared ancestors, especially when the same people are already clearly listed in both trees?

I'm pretty new to using AncestryDNA, so any tips for getting more out of ThruLines and DNA matches would be appreciated.