I just googled competitors and all of them feel way more vibrant + being more active on social media (I checked Instagram for the first two):
Building a slick landing page which is also responsive can be quite some work but looking at that competition it's a must, sorry. Look, it's not that people really need a monthly candy box, it's not a rational decision, it's something emotional—an impulse buy. So, if the landing page isn't even thrilling what should then trigger the buy?
Social media in particular Instagram might be a significant marketing channel because Japanese candy and its colourfulness is tailor-made for Instagram.
Maybe the business model is also the problem. There are little lock-ins/network effects and a low-barrier market entry for Japanese entities, thus all these competitors. There might be opportunities to pivot the business or extend into new fields/models. The first thing which comes to mind is licensing successful products for exclusive import/distribution to key western markets. Different game but highly scalable. Maybe there more unseen opportunities. If not then selling the business could be an option (after a final revamp re presentation and SEO).
However, Candy Japan has still one major strength: Frequently placing stories on social news sites like HN and Reddit which always hit top 10 positions and generate tons of free traffic (should be 20-30k visits per placement which would translate in 200 to 300 subs at 1% conversion).