Recently, I have been working with a lot of professionals where selling isn't their primary role. These may be account managers, recruiters, engineers, or project managers. They all have a mandate to do sales, but it's not their primary job nor do they like it very much as that's not the reason they got into the business.
However, these are busy, customer-facing people. They will call me and say, "Colleen, I'm too busy to do sales. I have project management work, client servicing work, etc." I'm not denying that they don't have a full day of work, but that doesn't give them a pass to neglect their sales responsibilities or that they can't do sales at all. In fact, I got a tip for you that will improve your sales, even if you're a non-seller.
All you need to do is ask a simple question every time you're servicing the customer: Is there anything else that you're working on that we can help you with? It works because the biggest reason your customer is not buying more from you is because they don't know about everything you offer. When you ask them what you can help with, it opens up the conversation. It also works because you're asking this question while you're already talking with the customer, so it doesn't sound salesy, it sounds like you're trying to help.