I use UTM parameters on all my links to track revenue sources, however, I see from my data that a majority of my revenue is coming from a 'blank' source which based on my analysis I think this is Google organic searches. I don't know why this is coming up as 'blank' because with all my advertising channels I always use UTM parameters. The only check I have to seeing what 'blank' is that I ask a "Where did you hear about us?" question during signup and a majority of users enter in Google; so this is why I believe this is traffic / customer leads is coming from organic Google searches. Should I then be dedicating more time/money on SEO instead of my other paid advertising methods since this seems like the channel that is bringing in my most revenue?
The short answer is "it depends, but more than likely yes." Investing in a managed an scalable SEO strategy can be the kind of marketing effort that pays dividends slowly (but surely) over time. If the validity and value of the organic traffic you are generating is high, and these customers are qualified buyers ready to make a purchase, then focus on growing the denominator (i.e. the pool of traffic from this source) is exactly what you should be doing right now. Generally speaking, as you increase in size the overall ratio of value to traffic size will diminish, but again you make up for that in the aggregate of a larger pool from which to drive purchases.
It's not so much a function of should you or shouldnt you, it is really more a function of to what extent you should be. I would say that (generally) investing in your SEO growth is a plan that will more likely generate medium-term results; if you need an immediate boost in driving sales then I would consider other campaigns such as looks-alikes on FB (if you have a good following there) or perhaps re-targeting for abandoned site visitors.
Are you utilizing and growing an email list from the people who visit your site? you might want to consider ways to monetize more of the numerator as well - in that if you can get email addresses with something like a lightbox roadblock pop-up form or something when visitors first hit the site you can set them up on a drip and drive re-capture opportunities back to your site.
happy to hop on a call and discuss further if you want to get into more details/ideas.