Peter's answer is solid for typical investors, whether lead or not.
I would ask whether you purposely referenced "LEAD investor" and "no longer LEAD your rounds". If you gave unique terms to a lead investor with some assurances they would lead in the future (not typical), then you could have baked in some "pay to play"/other requirements so they only retained their unique "lead" terms if they continued to participate in the future.
The typical venture track has a new lead with each new round, so it would be somewhat unusual to expect your prior lead investor to lead your next round. That said, unless expectations were set otherwise, it's not unusual to expect a prior investor to participate for their pro rata in future rounds. If they refuse you should get to the heart of why so you have solid answers for potential new investors.
If a prior investor cannot follow-on because they are structured with a smaller fund and specific investment caps over time, then you can sometimes explain away their non-participation so it doesn't torpedo future rounds (that is also a scenario where it may not be appropriate to bake-in penalties; consider "minor investor" versus "major investor" follow-on expectations/penalties). If, however, they are not following on because they have lost confidence in the company, that can be deadly. If that's the case, you should gather follow-on commitments from their co-investors in the prior round and spend extra cycles convincing them to remain supportive for due diligence and participate at some nominal amount assuming other, new investors will lead and take the bulk of the new round. If they won't do that, then you'll need to prepare solid answers to the questions they will raise (e.g. why they lost confidence) for new investors -- and encourage those new investors to decide based upon their own excitement for a deal rather than letting another, prior investor's unique circumstances cost them an exciting opportunity.
Lastly, if you do get new investors interested and they really want a prior investor to participate, the new terms could be so onerous that the prior investor decides it's better to invest alongside those terms than sustain a highly dilutive round with zero participation.
This can definitely be a thorny topic, but one I've seen/helped multiple entrepreneurs and syndicates navigate successfully -- good luck Frank!