Yeah, there are several aspects to ranking and a bit of a learning curve. The first step is simply to learn how the system works in general. You need to learn how to discover keyword phrases that people are searching for with little competition etc. While I agree with Damon I think there is an additional element here; you have hit the 'sophmore slump' IMO. You are ranking for these 'easier' tags and that is great, but often they are easier because there is less traffic. So... now may be the time to start setting your sights on slightly more competitive tags. Instead of aiming for the 'sure thing' 100 SEO Score rated tags, dip your toes into a few tags with a little more competition and try to assert your authority in your niche to take over ranks in a few of these searches. Maybe try one or two of these per video just to see if you can gain some traction against the competition.
FYI If/when you begin aiming for these more competitive search terms you need to make sure your thumbnail game is 100% on point. CTR becomes even more critical when you begin wading into the deep end.