Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
There is no ink, these are thermal printers, they use a thermal printhead that produces heat that appears as the characters on special thermal paper.
If you are using the wrong paper or it is installed the wrong way it can cause light print,
it may also be that the printers are dirty or worn out (these are old models).
You can clean the thermal printhead area (the metal bar where the paper feeds through) with qtips and rubbing alcohol.
If that doesn't help and the paper is the proper type and fed correctly, then the printer has just reached the end of life stage and isn't heating sufficiently. At that point the printer would need to be replaced, they typically are not repaired. I can help find a suitable replacement for you if needed.