When you combine topics, placements, and/or audiences in a single ad group, an impression has to meet all of the targeting types for your ad to be eligible to serve. In other words, Ad Exchange combines targeting types with a Boolean AND.
While combining these targeting types can help you hone in on your target audience, doing so also limits your reach. If you're not reaching your impression goals in an ad group with multiple targeting types, consider removing one or more. If you'd like to target impressions that match one or more targeting type (in other words, a Boolean OR relationship), create separate ad groups, each with its own targeting type.