You already provide discussion supporting your findings. Considering some of the points below might strengthen your discussion/limitations section (some of them mentioned in more details in later comments):
-Why analysis did not yield between-subjects differences
-Any other confounding factors you might've missed or can't control (e.g., factors related to the individuals or content or media)
-Expanding on how the controlled conditions might've affected how useful podcasts or both formats are (e.g., being in a class in front of others, knowing there's a test after, just getting this for 10 minutes, not being able to repeat or pause as you might usually do with podcasts)
-The inherent and design-related differences between the two formats (e.g., color coding and interactive element in skeleton concept maps), and how these might've affected the findings
-By extension, how those formats might have had subtle differences depending on the topic.
-lastly, since you state that the findings in round 2 might be because of carry-over effect, might be useful to dedicate a paragraph discussing why this would / or wouldn't be the case (as detailed below)