Under insured

What is the name of your state? PA

In 1994, I was 21 years old (1st Own policy). Insurance agent made me waive stacking of under insurance. Made sense I only had 1 car at the time. In 2002, bought a 2nd car. In 8/2003, was involved in auto accident(Other's fault) I had numerous back surgeries. I have $100,000 under insured for each vehicle but my insurance company will not pay out $200,00 because they say in 1994, I waived the stacking of it. I already received the max of funds from the other person's under insurance of $25,000

How can they get away with this? Shouldn't I have to sign a waiver in 2002 when I bought the 2nd car?

Steven