You don't say where you are? Climate? How old is the hen?
How about putting some buttermilk (which is also a probiotic/cultured) or yogurt mixed with some water to thin it a bit, into their food... for the flock?
The "pulsing" of the vent hole is normal.. look at the other hen's vents.. but, if she is "bearing down" and not laying, getting into the laying box/nest and not laying... staying there for long periods... keep on treating her as if she is eggbound... Search here on BYC for Eggbound and Eggbound Treatment and do the massage, warm baths for 20 minutes at a time.. OILED FINGER... stimulating and feeling inside the vent... gently.... you do not want to break any egg you feel in there. Repeat until she lays, poops, feels better... it can't hurt her it's a warm bath and a massage.
I ask where you are -- and the climate.... the days are shorter... hens lay less.. but, stopping completely can be something else. If you have a light on in the coop, or a light on a timer to extend their days... they should still be laying, some, some of them, laying less but still laying.
I got 10 eggs yesterday.. out of a normal max of 13 during the summer.. of course, I got 6 today... I am in Southern California where it is nice (65) during the day but frosting at night and I have a light in the coop on.
A couple of hens stopped laying completely, one for months and didn't start again until I had completed a thorough worming regiment -- wormed the whole flock with Valbazen. She had not laid since June and started again in early December after the worming.... after all traces of worms were gone.
Some stop laying when they are molting or sick or have worms or it's too hot or it's too cold... shorter days... but, stops (for climate or shorter days) are usually not completely stopped and not for too long.
You can also use the Search here on BYC to search "stopped laying".. or other symptoms.. the one symptom that you really can't use to search and get good results with is "lethargy" it's too common and too general...
And, what are they eating? Just laying food (layer pellets, mash or krackles)? Do you put some Apple Cider Vinegar in their fresh water? Could she have eaten something bad? Moldy food or ? Have you checked her crop? Massaged it, smelled her breath?
Depending on the ages of all of your leghorns... and if you haven't... I would consider worming... (with Valbazen... search here for "worming with Valbazen", "giving oral medications", "how much Valbazen", etc...) or, probiotics mixed in the food. Maybe... give your sick girl some water soluable antibiotics as well.... Duramycin 10 / Tetracycline... one tsp, or 1 1/2 tsp. per gallon of water.
Go to the feed store... your local feed store should have all you need -- do not ask for Chicken wormer or Chicken antibiotics.. most things we use are made for other animals and we just adjust and use off label for chickens. Others here know more than I do.. when you search for info on Worming, look for posts (more recent posts, recent years) by dawg53.... for best info on worming.
Good luck, I hope your searches and others will help you more...
