Sounds like a moisture issue rearing it's head. Better crawl under and take a lookat the subfloor from underneath.

Inject some white glue under the bubble, weight it down, and hope for the best.

Don't know what you mean by starting to bubble. Need more information.

I've looked in the crawlspace underneath and found nothing unusual. Seems dry.

The bubbling is areas of the sheet that have come unglued from the subfloor, typically round-ish areas about 1 foot in diameter. It all appeared all of a sudden, and it seems to be getting worse.

No matter how carefully it's installed, sometimes bubbles appear in vinyl for weeks, months or even years after installation. Trapped between the vinyl above and the subfloor beneath, the bubbles won't go away on their own. An everyday needle or utility knife and a rolling pin or floor roller are your secret weapons to spell each bubble's doom. Sweep and mop the floor to completely to remove dirt, food, grease or oils. Inspect the floor carefully. Look for small blisters and larger bubbles where trapped air and off-gassing pushes up on the flooring material. Pierce each bubble with a large, heavy-duty sewing needle, very small ice pick or the tip of a utility knife for larger bubbles. Spread a hand towel or similar cloth over the pierced bubble. Place a hot iron on top of the towel. Push down on the iron and move it slightly to avoid burning a hole in the cloth. Heat the flooring until the bubble is smooth and the vinyl adhesive is reactivated. Set a heavy brick, cast-iron frying pan, stack of books or another weighty object on top of the bubbled and heated area. Wait 12 to 24 hours before removing the weight. Inject vinyl adhesive into the bubble if previous steps fail to eliminate it.

Rarely vinyl with felt-like backings will delaminate and the result can be the bubbles (looseness) that you're noticing. The backing may still be bonded to the subfloor, but the vinyl layer may not be bonded to the felt. This can occur where the floor has been suggested to repeated rolling loads.

Is the floor a full spread product or just perimeter glued? You can generally tell by tapping on it with your fingernail starting the edge and moving away into the field. If the sound changes from solid to somewhat hollow as you get 4" to 6" from the wall, it's perimeter glued. Perhaps the shrinkage of the vinyl is causing the glue to let go in places and it's rippling up.

We had a floor guy come in and he claimed that the floor is actually perimeter glued, for what it's worth. We haven't tried to confirm it yet.

Also interesting, the bubbles have recently disappeared, I would presume related to cold weather. I don't know if that means the whole sheet shrunk and pulled tight, or if the air below the floor was drawn out (?), but at the moment the floor looks good as new.

We will probably wait until it warms up again and bubbles up again in the spring before we do anything.