My limited experience with making my own sets of series taught me to double check these few things.
Most importantly you need to start a new campaign for any new series you add to show up
Make sure the folders for all the stuff in the series are all in their respective folders in the data folder of the dtr2 install. ie: cars, series, skins and tracks
Make sure the car is the right one - snas super late model v2 (on page 2 of the chassis pages) I have had the wrong car more than once but now I check every skin and make sure it relates to the car I have for that series.
Make sure the names of the tracks in the series.str file are exactly the folder names of the tracks in your data/tracks folder if their is one not EXACTLY right dtr2 won't show the series.
Most importantly you need to start a new campaign for any new series you add to show up (Yes, important enough to say it twice )