Is Google sandboxing / penalizing my domain?

First time poster, hoping someone here can help. My URL is: Learn Conversational Spanish in 30 days

Problem: It seems like Google is penalizing my site, but I am stumped as to why. The site shows up fine if you do a site search for "onemonthspanish.com" but it is not ranking in Google for any sort of keyword-based search. My site ranks #2 in Bing and #1 in DuckDuckGo for the phrase "One Month Spanish" but in Google, the site doesnt show up anywhere in the first few pages of the serps.

Site Details:

• Domain is nearly a year old (new registration)
• I have had the current homepage for the last 8 or so months. I started adding real long-form content and a few manual backlinks about 2 months ago.
• The site currently has 30+ pages of "lesson" content that looks like this: One Month Spanish - Lesson 5: First day at work. The site has also 100+ short vocabulary pages like this: Teacher in Spanish | Spanish for teacher (and more People). There is also a blog, but only one article written so far.
• Google Webmaster Tools had indexed > 150 pages of the site.
• My Moz Domain Authority is 8, The Page Authority of the homepage is 20.
• Google Webmaster Tools recognizes 19 external links pointing to my site.
• The handful of links pointing to my site are all white hat, manually created links (a dozen or so blog comments, some links from my personal wordpress/tumblr blogs). Probably half of these do use the anchor "One Month Spanish" (which I assumed was the right way to do "branded" anchor text.)
• There are no manual actions or errors shown in Webmaster Tools

Is it possible that I have been hit with some sort of over optimization penalty?

My title tag is "One Month Spanish | Learn and Improve Conversational Spanish". My H1 is "One Month Spanish." According to SEOQuake, the phrase "one month spanish" is used on my landing page 5 times and has a keyword density of 1%. The word "Onemonthspanish" (all one word, no spaces) is used 13 times, with a keyword density of 0.87%. The word "spanish" is used 56 times with a keyword density 3.74%.

This does not seem like a lot, but maybe the combination of the title tag, h1, keyword density and anchor text all using "One Month Spanish" is causing it to get to get punished by Google? "One Month Spanish" is not a keyphrase that I actually targeting per se -- the only reason I am using it so frequently is simply because it the name of the brand/product. The terms I really want to rank for are things like "learn spanish", "conversational spanish", "learn spanish fast", etc but I am not currently ranking for anything in Google.

I'd be extremely grateful for any thoughts or ideas!