EMIS Population Reporting has a handy feature called Check Patient which can help to understand why a particular patient is or isn't included. There are two methods for using it.
It is possible to review how the patient passed each rule, and the final rule shown is ultimately why the patient was included. Helpfully, the code or medication will be shown, including the date to facilitate finding it in the record.
In the example below, rule 2 is looking for patients currently on the asthma register. Feature 1 finds the most recent asthma code (this is not the diagnosis date), and feature 2 finds the qualifying inhaler - it will usually display the most recent issue. Notice at the bottom of rule 1 (top of screenshot) the finding says "Patient moves to next rule", and at the bottom of rule 2 it says "Patient included". Any subsequent rules will not be tested.
Interpreting some of the rules can be confusing, especially if you didn't build the search. If you think the patient should be included check carefully the dates or codes (term and concept ID) found on the record against the search rules. Features will often show findings such as "No matches", "Found", or even "No matches, but included" (where the feature begins Exclude...).
Where there are multi layer searches, it may be necessary to review why a patient was included in a parent search in addition to the final search.
When a patient is expected to feature in a search but doesn't, it is still possible to use Check Patient against the highest level search the patient is not included in. Use the Check Patient icon on the ribbon: EMIS will automatically test any patient showing in the blue precis bar, otherwise it will open a Find Patient dialogue to load the patient required.

Running Check Patient on complex searches may take some time and EMIS may appear to be frozen.