Is there any way I can fight if I can’t get a severance package from a factory that is moving to Mexico?

I previously worked for the company for 18 years. I had taken another job offer which paid a lot more money and left the company on good terms. I ended up going back because the job I went to shut the doors. Now, this company is moving to Mexico and only wants to give me 3 years of severance pay even though I never took a pension payout when I left the company. The contract they gave us said we would get a week pay per year of service. It did not say continuous service. Just years of service. Between both times being with the company I have a total of 21 years. Like I said, they only want to pay me for 3 years.