Bill validators get updated to include newly designed bills, but in your case, the company went out of business long ago.
Yours will probably only work with bills from that era, and reject anything that has changed in design (it just doesn't know what they are).
Unless the unit is broken, a $1 bill should work, as it hasn't changed.
I'd just replace it with a newer bill acceptor.
By tracing our currency back to the colonial era, we can explore how American history has helped shape the way we design, issue, and process modern U.S. banknotes.
www.uscurrency.gov
2013
In its first redesign since 1996, the new-design $100 note features additional security features including a 3-D Security Ribbon and color-shifting Bell in the Inkwell. The new-design $100 note also includes a portrait watermark of Benjamin Franklin that is visible from both sides of the note when held to light.
Image: , CEP Website
2008
The new-design $5 note features subtle background colors of light purple and gray. The $5 note includes an embedded security thread that glows blue when illuminated by UV light. Two watermarks are featured in the $5 note, which are visible from both sides of the note when held to light. A vertical pattern of three numeral 5s is situated to the left of the portrait and a large numeral 5 is located in the blank space to the right of the portrait.
Image: , CEP Website
2006
The new-design $10 note features subtle background colors of orange, yellow, and red. The $10 note includes an embedded security thread that glows orange when illuminated by UV light. When held to light, a portrait watermark of Treasury Secretary Alexander Hamilton is visible from both sides of the note. In addition, the note includes a color-shifting numeral 10 in the lower right corner of the note.
Image: , CEP Website
2004
The currency redesigns continue with the $50 note, which features subtle background colors of blue and red. The $50 note includes an embedded security thread that glows yellow when illuminated by UV light. When held to light, a portrait watermark of President Grant is visible from both sides of the note. In addition, the note includes a color-shifting numeral 50 in the lower right corner of the note.
Image: , CEP Website
2003
The new-design $20 note features subtle background colors of green and peach. The $20 note includes an embedded security thread that glows green when illuminated by UV light. When held to light, a portrait watermark of President Jackson is visible from both sides of the note. In addition, the note includes a color-shifting numeral 20 in the lower right corner of the note.
Image: , CEP Website