It can be slow getting them. It sounds like you are doing pretty well. I’ve found it goes in cycles. I think that the companies switch marketing strategies ever so often, and if you hit them when they’re going for word of mouth, you score.
How active are you on various social media? I find that’s one of things that often gets mentioned when a company contacts me – they saw my post on FB or Twitter etc. I am friends with every company on those sites that I can think of, and I respond to their posts. I share and retweet if the post is anything other than a direct sales pitch.
I have contacted companies and asked to review an item, but most of the time I just share my artwork. If I already use any of their products, I review them (with artwork) and post about the review on their social media sites. It doesn’t always pay off. I never got Prima’s attention even though they featured a couple of my watercolors. But I do get hits.