You have 2 issues:
One is production samples versus lab samples; the other is your MSA design. I'll tackle that first.
It does not appear the experiment is sufficiently nested.
You can never ever separate the variation in the samples from the measurement system variation with destructive testing because you cannot recheck the same part the way you can with a gage. You need to construct a different experiment to remove or identify as many of the sample related variables to minimize the test variation and sample combined.
How many samples can you make from one seal?
How many seals can you make at one time - ie is this a continuous process or is this a single or multi-cavity tool that makes the seals?
How many seals can you make from one lot of material (ie - substrate, adhesive, coating all the same lot)
There may be others, but I don't know enough to speculate further than this.
For those of you who have participated in round-robin testing between test labs for any given test, the solution to this MSA is very similar.
For the first part - why can you not just use lab parts?
This is where so many practitioners go astray. While it is true, you can improve your MSA results under more tightly controlled conditions, you may end up missing some significant factors affecting variation within or between samples that you might otherwise detect when you use real production samples.
MSA is to determine if your measurement system is adequate to make decisions for one of two questions: Can you get a reliable judgement of good or bad with minimal risk of a wrong judgement based on the measurement? Is the measurement system suitable to detect process changes when used for statistical evaluation - ie SPC and or Capability Analyses?
It appears you may have reason to question the test method and/or the process making the seal. I would recommend you perform a lab sample MSA to evaluate the measuring process relative to the results you obtained so far. Again, you will still need to develop a more rigorous experiment to analyze the components of variation.
If you need help with either of these, let me know.