Main/subreport group linking

I have a main report and a subreport linked. Both reports are grouped by field tbnatur.desc. Basically, this report compares calls for service for one year (main report) compared to another year (sub report) and the tbnatur.desc field shows that the call nature is. The problem is that sometimes, for example, a call nature may not have been used in 2019 (main report) but was used in 2018 (sub report) but since that row does not exist on the main report, that result from the subreport will not show up on the main report. However, if that row exists on the main report but not on the subreport, the row will display and the value for the subreport will just be blank. That part is fine. Basically, I want to blend the subreport into the main report. If that row does not exist on the main report but does on the subreport, I want the result from the subreport to be on the main report.

I have attached a screenshot of the report that I hope helps explain this. The column on the left is the subreport results and the column on the right is the main report results.

Here is a screenshot of the subreport when ran by itself.

I do have the tbnatur.desc fields linked however I am still not getting the results I need. Any help would be appreciated. Thank you.