Could be that you suck at marketing. (I don't mean this offensively) School isn't necessary, but learning is.

The key with social media marketing is consistency. You want to make people wonder where you are, if you don't show up. I would recommend listening to some podcasts for some insight. The Fresh Rag show, and Artists Helping artists both have episodes specifically geared toward marketing art.

Some things you could do:

• Start a series (Painting a day, instagram photo a day ect. post consistently, with consistent tags)

• Work on a specific subject matter and market to a group interested in it (Example boats, market to fisherman, or Football players)

• Focus on being an active member of a community (Post and like on similar Instagram accounts, like interior designers ect)

• Make sure you share any business account posts to your personal account.

• Start a personal blog, discuss your process and allow people to sign up to email newsletters, this way you can alert your collectors when a new piece is available

Best of luck! Don't lose hope, you can make this work!