Your toothbrush is worn-out!

Probably depends on which body you are using. In mine it both flashes a tiny LED and does an extra little buzz-buzz buzz-buzz signal with the ultrasonics when you turn it on (or off, can't recall which) when the head has 'expired'. Continues to work fine, though. You might be interested in this YouTube video from Applied Science with electron micrographs of 'new' and 'worn' toothbrush bristles - there is a very marked invisible change that happens: ▶ this video

Something else you could send: >Does dentifrice use help to remove plaque? A systematic review link "The cumulative evidence for this systematic review demonstrates that there is moderate certainty that toothbrushing with a dentifrice does not provide an added effect for the mechanical removal of dental plaque." I also don't use toothpaste. However, I do replace my toothbrush regularly (every 3 months, which I believe most dentists recommend). Toothbrushes have microscopic texturing to help them clean, which wears out with use. Here's a video showing it under an electron microscope: ▶ this video A fresh toothbrush cleans noticeably better than a worn one. I think this is much more important than buying fancy expensive toothbrushes. Even the cheapest toothbrushes clean well when they're fresh, and the expensive ones wear out just the same.

The usual problem with toothbrush bristles is they become microscopically worn down, so they don't scrape off plaque as effectively. Even if you can't see the problem, it might not be working as well as a fresh head will. You can find electron microscope scans of fresh toothbrushes and worn ones in this[0] Applied Science YouTube video. [0]: ▶ this video

I use an even simpler and more reliable system to avoid running out of toothpaste: brushing with plain water. Evidence suggests that brushing with toothpaste (called "dentifrice" in scientific papers) does not remove any more plaque than brushing without it. Here's a systematic review from 2016: >Does dentifrice use help to remove plaque? A systematic review link "The cumulative evidence for this systematic review demonstrates that there is moderate certainty that toothbrushing with a dentifrice does not provide an added effect for the mechanical removal of dental plaque." However, plaque removal is not the only purported function of toothpaste. It can also serve as a fluoride delivery system, and fluoride has strong evidence for strengthening teeth. I personally drink a lot of tea, which is a fluoride hyper-accumulator, so I'm more concerned with avoiding fluoride, but if you don't drink tea this might influence your decision to use toothpaste. There are also some toothpastes with antimicrobial ingredients. Those also have strong evidence for actually doing something, although I'm personally skeptical that messing with your mouth's microbiome like that is a good idea. Additionally, I believe that toothbrush wear is an underestimated factor in cleaning efficacy. Toothbrush bristles are manufactured with microscopic texturing that helps remove plaque. I can feel the difference when changing from a worn to a new toothbrush. Ben Krasnow of Applied Science has a video on Youtub