I've had the same problem. On one I sprayed the handle with lubricant and that worked fine, on the other one, we wrapped it with heat tape dug down a foot and put heat tape down as far as possible and that has worked all through my bad winter. I was going to cover the heat tape one with a trash can but never got around to it. It hasn't frozen since.
I have an outside hydrant that did that.It was freezing in the head due to the plunger thing being worn out.I could pour a pail of hot water over the whole thing and thaw it out.When I shut it off I was ok if it did not drip out a drip once in a while.I also put a bunch of pea rock around the bottom of the hydrant when I dug it up.
I would disconnet the hose after each use, and as you said, guess how I learned that one a number of years ago.
I have heat tape wrapped and taped to the pipe from the top to the drain hole. Left it on for a week and did not thaw out. However, I did not cover with any straw, etc. Will cover it with straw/etc and try again.
Dumping hot water down the pipe worked for a few days, then froze up again.
I am leaning towards the plunger being bad, but very much welcome any and all other continued ideas to look at.
Remember the good times, for they are fewer in number and easier to recall.
Ours was doing this at the start of winter. We thawed it with a weed burner a few times. Finally pulled the guts and replaced everything, the plunger was scored but didn't appear really bad. It froze once more and I bought a jug of RV antifreeze with plans to shoot some down the pipe with a big syringe and some tubing. But before I could, it thawed and worked the rest of the winter. I also shot some compressed air down the hydrant to blow out the drain hole if that was the problem.
Last time I put in a hydrant I used a 4" PVC pipe to surround it, shot it full of foam insulation and passed the pipe at the top through a PVC cap with a hole in it. Finished filling the voids with foam from there.
Fact of life is metal is a good conductor of heat. If the frost level is 3 feet deep heat will attempt to follow the metal to the cold layer. If that cold level starts where the PVC pipe ends the heat from the valve has to travel an extra 4-5 feet.
There is a broad gambit in the design and the use of the materials that various manufacturers use in building hydrants. For example the best hydrants will have a non corroding stem. Others will have a steel shaft that will rust and bind. Box stores sell where the margin for the store is best. Old established plumbing supple houses tend to sell two types, the good and the cheap. It pays to be an informed buyer and there is a lot of information on the internet regarding yard hydrants. If the hydrantrant is outside then a 5 gallon bucket inverted over the hydrant will stop a lot of rain water from freezing inside the riser pipe.
I had 2 hydrants freeze up at the beginning of the winter season. 1st one is in the horses loafing shed and I left the hose attached (da!) I thawed it out right away with a blow torch, kept the hose off, and it has worked fine since. The 2nd one is really old and completely out in the elements...no wind block or anything. It froze up right away when it got cold and I wasn't surprised. Then we got a mini thaw and it was working again so I started keeping a bucket over top of it...never froze up again. Amazed me when I trudged out there when it was 30 below (real temp not wind chill) and it kept on working!
We have quite a few water hydrants. It's very convenient. But.... this year the one most protected in the barn froze up. They are supposed to be unable to freeze but we figure that it somehow didn't drain as it should. Grrr!
Moms don't look at things like normal people.