"I use cannabis for back pain. Should I be only concerned about THC and CBD, or should I be looking for plants with the right terpenes as well? If so, which ones? Thanks for your help!"
If the variety of cannabis you are using isn't addressing your pain, first make sure that both CBD and THC are present. They work against pain together better than either one does alone: 2+2= > 4. Also make sure you are not using too much. THC is most effective against pain at low to moderate doses and is very ineffective against pain at high doses. You might look at the terpenes associated with pain relief, including muscle relaxation, and decreasing inflammation. All of these modalities play a role in causing and perpetuating pain. Caryophyllene (black pepper, cloves, oregano, rosemary, cinanamon, basil) is a pain reliever, muscle relaxant, and anti-inflammatory; linalool (lavender, citrus fruits, coriander, bay leaves) is a pain reliever; pinene is an anti-inflammatory and pain reliever; myrcene (mango, bay leaves, hops, thyme, lemongrass) is analgesic and anti-inflammatory and humulene (hops) is a potent anti-inflammatory. Don't limit yourself to the terpenes in cannabis, but also consider incorporating these terpenes into your diet and aromatherapy as well.
Cleaning houses for a large part of my life has led to back pain for me. I recently started adding the Lime OG terpene from https://www.true-blue.co/products/lime-og-terpenes to my vaporizer and I do feel a difference.