I'm a professional disc golfer. Discs are strictly regulated by weight limits. I have discs that are many years old that have actually lost weight/mass. One disc initially weighed 174 grams and now weights 165 grams. Do you know of a liquid that could be absorbed by the disc to gain weight?
1992 pro master world champion
I have never heard of disc golf, so I do not know how the discs are made. My guess is that they lose water over time. You might try soaking them in water to restore the weight.
For plastic discs, they may be losing plasticizer--just like automobile dashboards, steering wheels etc. dry out and crack over time. I would talk to someone at an auto parts store or auto interior repair place to see if they have a product that could be applied to your Frisbee-a conditioner type product. I've heard of using olive oil instead of an "Armor ALL type product" on cars but I have personally never tried it.
Like others here, who have commented, I am unfamiliar with disc golf discs. However after a brief Google Search, I did learn something of the composition of the dics used for this purpose. You should have been more specific as to anything you know about the disc in question, the brand, the possible composition, etc, to better enable those of us chemists who don't know discs, but do know something about plastics to better respond. Perhaps you could add to your commentary by anything else you know about them.
From my limited on-line reading, I learned that discs are generally made of thermoplastics and are injection molded for the most part. Many dics are said to consist of virgin plastic, plus recycled plastic in some proportion. Types of plastics cited were polypropylene, nylon, Hytrel (DuPont tradename for a polyester), and urethane. There may be others.
I am assuming that the physical integrity of the disc is such that there is no visible loss of material or dimension. Is this the case? If not and if there has been a weight loss and in this case a substantial one of 5.1%, it is likely that this loss is of a material added to the plastic in the thermoforming process to improve properties. Such materials are usually low molecular weight compounds and are termed, "plasticizers."
Of the various materials, which I found listed for use in disc golf discs, here are some quick thoughts about plastic properties and plasticizers.
You can read more about various plasticizers in this pdf document found here: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwjl5OfmtargAhUEWqwKHR8ZC60...
With the exception of maybe water, it would be difficult to impossible to reimpregnate a thermoformed plastic product with a plasticizer after the fact. Without more information, then, I would suggest that you soak a disc at room temperature, weighing it before and after a soak of a week or more to gauge if there has been any weight gain.
Alternatively, you might also try one of the automotive conditioners suggested by Barbara Bossard above weighing before and after to see if any weight is gained.
If not, then, I suggest abandoning those dics.
Hope this helps.