How do I get my sales team to use the content that I produce?
There needs to be alignment on the content itself between you and the sales team. Is the content answering a pain point they have? Is it actually useful in answering questions their audience has? Are you prescribing or pushing a specific time and place to use the content? Is the content actually effective in answering the question (it might need some better design or copy for conveyance). It helps to also see the different hacked-together materials they use and ask them why they use those "black market" slides / content.
I periodically survey my sales team and ask them what their most challenging conversations are with buyers, what materials they have found the most helpful and WHY, and what their most burning need is. When I push them new materials or highlight something I've already sent them, I specifically reference the survey results whenever possible. And whatever the biggest request is, I make a point of delivering ASAP, again connecting the dots for them: "You asked me for this, and now I'm delivering it to you." This approach has yielded many benefits, not the least of which is credibility with my sales team.
I recommend collaborating with your sales team in producing the content. The best way I've gotten my sales force to care about what I produced is to align with what they need and possibly produce themselves. So if they work on decks often (which they likely do), how can you help them by incorporating your messaging, market insights and feature success stories in their decks. Proactively create one and ask for their feedback so you can tailor it to be something they would likely use. Then find out what they send in follow-ups and see how you can help there. Eventually when you create GTM packages you can align with them to ensure you're solely producing content they would yearn for and leverage immediately. This helps you become more valuable to them and prevents you from wasting time on collateral they are not using.
I agree with what others have said and would add that you might benefit from a bit of a retrospective with your main sales counterparts. If they're not listening to you or don't care about what you product, you may have earned a negative reputation with them. Repairing that reputation will help accelerate the adoption of whatever new approach to your work you end up taking.
Having one or a series of retrospective conversations with them and answer questions like "what are the main areas for improvement you see from the work I've produced over the past 3 months," "what could I be doing to be a better partner to your team," etc. You don't need to act on every piece of feedback they provide, but they'll appreciate the chance to be heard. Further, it's important to know exactly what wasn't working before you try to change up your process. A huge part of product marketing is influencing without authority, so having a mindset of "how can I be a better partner to my stakeholders" rather than "how can I make them listen to me" can be very beneficial.
Your issue is credibility. Salespeople will listen and use your content if they believe your content will move their deals and help them hit their quotas. Two things you should do: (1) get on calls with them so when you conduct training you reference a call where you feel your assets will help. (2) you should leverage two or three members of the sales team who are high performers and leverage them to test your assets and messaging first and leverage them in training as evangelists. You can even include them as mentors and judges in the certification process.
Include your sales team from the beginning and you'll largely remove this problem from the root. Orient yourself around what's in it for the sales team and they'll sit up and start to take notice.
I meet with the leaders of sales and SDR teams each week to surface problems that they're having and review pipeline metrics. Then we design problem statement together which our marketing team uses in an agile fashion to determine backlog and prioritization of projects.
When I share the finalized content with the sales team, I tie it to the problem that we're aiming to solve for them. This has fundamentally altered the nature of the relationship between sales and marketing for us.
Be sales' therapist. Really. Talk to them. Understand their pain points, and, more importantly, get them to have more clarity about their own problems and needs. You don't "get them to use (y)our content." You get them to use your concepts by soliciting their input and using it to develop content that actually resonates with them and will help them close more deals.
Product marketing's value is in the concepts it develops; the content is simply a vehicle for conveying those. So, make sure it meets the required specs of whomever will be driving it.
Feedback from champions in recent deals can be helpful here, too. When was it hard to make the case internally to switch to your product? How did any competitors in the deal make it easy to buy from them than you? Correlating the buyer's experience/ feedback with your won and lost outcomes should help both you and sales make objectively better decisions.