I have experienced similar problems in the past and tried few things to get the seniors on board.
What I found most effective was if I am aware of a certain problem that needs to be improved but there is a lack of something (motivation, money, man power etc. kind of excuses)
1. Every chance I get, I mention how necessary it is to do this and if we don't do it auditor may pick this up and it can lead to lower grades in audits (because even though they lack commitment, most senior managers care if the audit result is bad).
2. During 2nd and 3rd party audits I kindly introduce the problem to auditor and ask him to include in his non conformance report, whether as an observation or as a non-conformance and make sure the senior managers are in closing meeting so they can hear from the auditor what I have been warning about.
This usually works or accelerates the solution.
However when it comes to implementing changes in FSMS I do it a little bit differently. Usually the foreman or experienced employees resist the changes which ends up production manager backing them. Because from their point of view every change I want to implement slows down the production. So before introducing the change I spend extra time on the floor with the employees who are likely to oppose the change and try to find a middle way. Usually I start by introducing the problem to them, and ask them what they would do to solve it, what changes they would implement to solve it. Very often they come up with very similar solutions if not the same one I thought of. It had happened that they came up with far better solutions than I could think of so it is beneficial for me to do this exercise anyway. After consulting with few of them and get their opinions on how to implement a change I wait for a chance to discuss this together. Sometimes I go in the coffee room during their break and start the discussion, let them discuss between them pros and cons of their solution while introducing mine. It is important to do this in a relaxed environment, in a friendly way. It is a bit of psychological game.
After all this;
1. if what I wanted to change is similar to what they come up with I go ahead and introduce the change. It is their idea not mine. So there will be minimal resistance.
2. if their ideas are far away from the solution, then I try to change the procedure in a way to include some suggestions from them so that they will feel part of the change and resist less.
I think as a general rule it is a very good idea to spend more time on the floor to interact with experienced employees to get a feedback from them also to influence them. If they feel they are important part of the FSMS they will tend to own it more.
Note: I always worked in a small scale companies (45-100 people) I am sure it would be different in much larger companies.